I too find a great deal of stress and anxiety relief in wearing diapers, etc. While I only wear occasionally the stress relief and mental relaxation is a big part of it and when I can't do to because of RL commitments, travel, etc. my stress and anxiety levels noticeably increase.
So I definitely understand that part. I also understand your instinct to say "this helps me" and that instinct probably comes from a good place on your part but I would be very hesitant to mention it. For one thing, and this is a general observation about the world in general and not at all a reflection on your concern or compassion for your friend, it can be very frustrating when everyone wants to pop up with "this one weird trick doctors don't want you to know about" style suggestions for your mental health issues.
Secondly, it's not like we're talking about a breathing exercise here. In order to make this suggestion you have to put yourself pretty far out there in revealing some very personal things. That can be very touchy for obvious reasons. It's also a little dicey because even if you're willing to do that and are confident in doing so, it can cause additional stress for your friend as if they choose to reject your suggestion it may feel in part (either to them, to you, or both) that they are rejecting you simply because what you've brought up is so personal and important to you.
Finally, it feels like a pretty long odds shot that they will suddenly discover some latent AB/DL tendency. Sometimes long shots come in but given the above I'd think twice about it.
It's once of those things where once you've put it out there it is out there and cannot be gotten back so you want to be pretty darn sure that is what you want before you take the plunge.
Lastly, of course, if their anxiety is severe enough to be causing them that much distress and you this much concern it probably suggests professional help, or at least a conversation with a professional, is warranted if that hasn't already been done.
Sorry, I don't know why I wandered off into all that. As I'm rereading your post you didn't actually say you were thinking about bringing it up, just that you wanted to, which is of course a bit different. That was just how I read it so I apologize if this response isn't at all what you were looking for.