How important is trust in a relationship

"Hey guys, you won't believe what's going on with a friend of mine and his girlfriend. They've been dating each other for six months now, and my friend really loves her. But they had this big fight a few weeks ago, and she went to visit her mother to cool down.

While she was gone, my friend was so depressed, that he ended up hooking up with some random girl he met in a club.

Anyway, a few days later, his girlfriend comes back, and she finds this girl's thong panties in the bathroom, and she KNOWS this thong isn't hers.

So she confronts my friend on this, and he lies and says that the panties are his! And that he likes to dress up in women's underwear. So I don't know if his girlfriend knew he was lying and just wanted to punish him, or if she really is into this or not, but she said she thinks that's really kinky and wants him to wear women's underwear around the house.

So he's been doing this for a few weeks now and is absolutely… MISERABLE!"

"So I think he should just come clean and let his girlfriend know what happened. What do you guys think? How important is trust in a relationship? Or do you think some things should remain hidden, even if it means being miserable?"