This is a touchy subject. Men and sex go along with egos. Every man wants to believe that every time you and he has sex that he’s doing his job but that’s not true.
You need to have a private conversation with your girlfriend. What was her angle in saying that to your man? I would definitely stop sharing that type of information with her. It’s very close friends you can barely talk about sex with your man with anyway. Be careful who you confide in.
Be truthful with him as the cat is out the bag now. Let him know you didn’t mean for it to come off insensitive but that you were talking to a girlfriend you thought you could trust. Let him know that you love him and that you aren’t attempting to down play his bedroom skills. Give him time trust me every time you have sex with him he’s going to think about what she said.
Hopefully with time and space he will be okay. Always be honest with your partner about sex from the jump. There’s room for improvement for all the key with men is not to burst their ego while being honest.