DEAR READERS: I received a letter from a woman signed "Feeling Rotten," who was beside herself with guilt because while visiting her best friend in another city, her friend's live-in lover came to her bedroom in the middle of the night and she "let him" make love to her. She admitted that she responded positively, adding she "hopes" she's not pregnant because they used no protection. "Feeling Rotten" asked me if she should tell her friend what took place. I aired the pros and cons, then asked my readers how they would vote.

So far, it's a dead heat - and the letters are still coming in: Some responses from readers who say "Tell!":DEAR ABBY: "Feeling Rotten" should feel rotten, because she is rotten to the core. She must have given her best friend's live-in lover some kind of encouragement - flirting, body language, eye signals to let him know she could be had - or he never would have had the chutzpah to come to her bed in the middle of the night. Yes, she should tell her friend what kind of bum she's living with. It will probably be the end of their friendship, but if she keeps quiet and her friend marries this heel, it could be the worst mistake of her life. - BEEN THERE IN WILMINGTON, DEL.

DEAR ABBY: "Feeling Rotten" should tell her friend immediately. Also, both she and her friend should be tested for AIDS. If they test negative, they should be tested again in six months. And it goes without saying that the sleepwalking boyfriend should also be tested. - AN M.D. IN THE BRONX

DEAR ABBY: "Feeling Rotten" should tell! How I wish someone had told me what a louse my husband was. He was in his last year of medical training and it was accepted that his hours were erratic and his schedule flexible. Finding out from a phone bill with a list of 15 of the same numbers from a different city was the most agonizing revelation I had to face. He swore he'd never do it again. But he did. If only a friend had told me. I later learned that everyone knew it but me. We are still married and are reasonably happy, but I'll never be able to trust him. We have two fine children, which is the cement that has held us together. - NO NAME OR TOWN, PLEASE

DEAR ABBY: Here's what I would tell Miss Feeling Rotten: Tell your friend everything - exactly the way it happened - then accept the consequences of your friend's reaction. If your friend is so hurt she no longer wants you for a friend, then so be it. I hope you realize that your friend is feeling twice the pain from having been betrayed by both her best friend and her lover. - PAUL IN PHOENIX

DEAR ABBY: I went through exactly the same thing "Feeling Rotten" experienced, only I cheated with my best friend's husband. When I finally couldn't stand the deceit anymore, I confessed. Do you know what she told me? "What took you so long? It's been all over town for months." - FORMERLY EASY IN PHILLY

DEAR ABBY: This happened to me. Only I was married at the time. My wife had a younger sister who spent the night with us. One night my wife went to bed early. Sister and I stayed up to watch television. She said her back ached, so I rubbed her back. One thing led to another, and we ended up in bed. We both felt so guilty, we went to my wife together and told her what had happened, and promised it would never happen again. She was hurt, but she forgave us, and we all put it behind us. Everything is better out in the open. Today, I have no desire for the sister - nor she for me. (She's married now.) Confessing took the "romance" out of it. - NO MORE GUILT IN VERMONT

DEAR ABBY: Why are you surveying your readers about something that has only one answer? Of course "Feeling Rotten" should tell her friend that the man she's living with is sleeping around. Women should stick together. Why keep quiet and protect a cheater? Since it takes two to cheat, they are equally guilty. Those two unprincipled dimwits deserve each other. - CHICAGO SCHOOLTEACHER

"How to Have a Lovely Wedding" is a revised, up-to-date guide for formal church weddings, home weddings, second-time-around weddings. To order, send your name and address plus check or money order for $2.89 ($3.39 in Canada) to: Abby's Wedding Booklet, P.O. Box 447, Mount Morris, Ill. 61054. (Postage is included.)