I think the bigger problems here are that he is stealing, he is crossing boundaries which should be clear and there is a strong possibility that this is beyond `fetish` and starting to edge into a disabling obsession.

I mean, he could be developing relationships with actual women his age who either wear underwear he would think was cute or think it was fun that he was keeping theirs... but he isnt doing that.

Instead he is eschewing real realtionships for the option of being alone.

There are a few grey areas there: does everyone need to be in romantic relationships constantly? NO. But he is clearly needing SOMETHING for comfort, companionship, and all that... whatever, and chosing to fulfill that need with some imaginary function.

by doing that, he is giving into very questionable moral decisions which are very likely to eventually end with him getting some kind of sexual offense charge slapped on him if he is not careful- and it sounds like he really isnt at all.

He should seek help, and thats not a judgement, to be clear- because panties certainly are fun. The issue is not the panties.

The very real root here is that it sounds like this could be approaching the point where it isnt healthy and in the very least he is experiencing serious lapse of judgement on a regular basis and should talk to someone about THAT if nothing else.