After months, even years, of talking to drug rehabilitation groups, Nancy Reagan has shown how she goes one-on-one on drug abuse. A student at Bishop McNamara High School in Forestville, Md., called her up at the White House the day after she talked about it at a school assembly last week.

His problem: How to get help for some of his friends using drugs.

Her solution: Send him some literature and offer to provide professional help.

"He wanted to protect his friends and did not want to rat on them," said Sheila Tate, the first lady's press secretary. She did not identify the youth.

She said the first lady told him that some day his friends would be grateful and that "I can imagine how proud your parents are." Later, according to Tate, the White House called McNamara's principal, Brother Walter Kramar, to "alert" him about the call.

At the assembly the previous day, Mrs. Reagan had more thoughts about how a high school drug user might get help. "I think they would first go to their parents--there are many parents' groups now forming--and then that their parents hopefully would contact a parents' group or would take them to their pediatrician," she told one young questioner.

The McNamara youth calling her at the White House first was screened by the first lady's special projects director Ann Wrobleski. The way the first lady saw it, Tate quoted her as saying later, the whole thing might have been a scene from her forthcoming appearance on the TV show "Diff'rent Strokes."

In that episode, which airs in late March, Arnold (Gary Coleman) submits a story about drug abuse to his school newspaper but the principal refuses to publish it because the youth won't reveal his sources. Mrs. Reagan drops by Arnold's house that night and the next day accompanies him to school.

The first lady was unable to meet personally with the McNamara youth because she was flying to Arizona the next morning to help her mother, Edith Davis, move. From there she will fly to California for the West Coast tour of Queen Elizabeth II and Prince Philip. She will tape the "Diff'rent Strokes" episode after the royal couple departs.

For all you commoners--nearly 1,000 of you--invited to see the queen at various dinners, lunches and receptions scheduled along the royal road show that opens Saturday in San Diego, here's a tip on the dress code: Don't worry about wearing black--although black with a touch of color would be cheerier.

U. S. Chief of Protocol Selwa Roosevelt, who will accompany the queen and Prince Philip on their 10-day West Coast odyssey, says she prefers to wear colors but she does have a black dress in one of her suitcases because it's a favorite. She'll probably wear that to a private dinner being given for her and her husband, Archie, when they precede the queen to San Francisco.

Back in "Old Blighty" nobody would be caught dead wearing black in the presence of the queen. It's considered too sad and mournful.

California, on the other hand, isn't "Old Blighty," and nobody expects those who live there to change their ways for the queen of England any more than they are expected to curtsy to her and bow to her prince consort.

Since invitations started arriving, however, there has been some confusion about dress-code terms.

"Californians have learned that 'lounge suit,' for instance, means business suit, not a masculine version of lounging pajamas," says one repatriated Los Angeleno.

"Out here, 'informal' means jogging suit and Adidas," says another, "but I'm sure that the night of the dinner at 20th Century-Fox everybody will rise to the occasion and wear dark suits and long dresses. One thing is certain: There's not a woman in the crowd who'd dream of being underdressed."

And straight from the board room of E. I. du Pont de Nemours & Co., a little insightful libretto on the state of the U.S. economy. It features Edward G. Jefferson, Du Pont chairman, Gilbert & Sullivan devotee and chemist-turned-sometime-songsmith who confesses that "occasionally with a can of beer, my wife and I will make a little music."

With wife, Wunny, at the Luxembourg Embassy's concert grand the other night, Jefferson showed that his stuff isn't all chemical. The Jeffersons' gig followed one by Adrien Meisch, concert pianist as well as ambassador of Luxembourg, and his wife, a well-known Embassy Row thrush, Candace.

Borrowing from the "Mikado," the British-born Jefferson provided an economic forecast for the Meischs' black-tie guests, who included Deputy Secretary of State Kenneth Dam, Assistant Secretary of State for European Affairs Richard Burt, Associate Attorney General Rudolph W. Giuliani, Sen. Charles Percy (R-Ill.), Chief of Protocol Roosevelt and former Ambassador to Luxembourg Kingdon Gould.

(To the tune of "Willow, Titwillow")

"On a tree by a river Mr. Secretary sat

"Singing 'Willow, titwillow, titwillow.'

"And I said to him, 'Donald, oh, why do you sit

" 'Singing "Willow, titwillow, titwillow."

" 'Is it prospective tax cuts, Donald?' I cried

" 'Or a budget too fat for your little inside?'

"With a shake of his poor little head he replied

" 'In spring things will bust out all over.'

"He smacked at his chest as he sat on that bough singing

" 'Bob Dole and Dan Rostenkowski.'

"And a cold perspiration bespangled his brow

" 'Help! Volcker. Help! Senator Percy,'

"He sung and he sighed. A gurgle he gave

"And he plunged himself into the billowy wave.

"And an echo on a suicide's grave:

" 'In spring things will bust out all over.' "

"Now I know just as sure as my name isn't

"Willow, titwillow, titwillow.

"That 'twas high cost of money that made him exclaim

" 'Oh, willow, titwillow, titwillow.'

"And if Congress and Volcker are obdurate I

"Shall perish as he did and you will know why.

"Though I probably shall not exclaim as I die

" 'In spring things will bust out all over.' "

All within earshot were economically appreciative, especially the guest of honor, Wellesley-educated Colette Flesch. In addition to being the former mayor of Luxembourg City and the current vice president of her country, Flesch is--at no extra charge--minister of foreign affairs, minister of justice and--appropriately--minister of economy.