Even if it were true that all men will cheat (which I don't think it is) has your friend considered that an attractive young nanny will almost certainly have zero interest in her husband? I strongly doubt he's the irresistible catch your friend seems to think he is.
If she wants to believe that all men will cheat given the chance, that's up to her. She's entitled to hold that view. However, think the part that's irritating is the fact she assumes she is correct and then patronised you. lt also sounds as if she wants other women to be as insecure as she is, because that makes her feel as if her own jealousy/distrust re. her husband is normal.
Nobody can ever be 100% certain what another person might do, even if they know that person really well, so ultimately we all have to take it on trust, and I definitely, definitely trust my DP not to cheat. If I didn't trust him, we wouldn't be together. I couldn't be happy in a relationship if I had so little trust in my partner. If I thought all men would cheat given the chance, I'd honestly have chosen to stay single all my life.
FWIW, I've certainly had conversations with male friends and colleagues about this kind of thing, and their views and feelings about the subject differ wildly. I used to regularly go out with a group of colleagues in a past job and one of them openly admitted to cheating on his wife and defended himself by saying all men would do the same, and one of the other men in the group had a proper go at him and 'No they wouldn't, so don't use the rest of us as your excuse to treat your wife like shit'. And my brother (who's single) was moaning the last time I saw him about someone he works with who has been boasting about cheating on his wife. My brother said 'Obviously I can't tell him not to cheat on his wife, because it's none of my business, but I did tell him he'd make a big mistake in thinking I'd be congratulating about it when I actually think he's being a disgusting human being.'