Going to professional would be a good thing for your friends going through this. On the other hand, and this isn't saying your friends are like this, I had a friend that tried to fake having disassiative personality disorder to try and get attention.
It's been a long time since my psych classes, but I seem to remember something about physical symptoms during a change over, kind of like a full body hiccough in some cases.
As for worrying about them hating themselves, I think that would be less about the disorder and more about the trauma which was severe enough to enduce it. Think of the disorder as (likely) simply the symptom of a greater problem.
And, like I said above, some times you need to be critical of your friends. Obviously, I don't know them, but it can never hurt to seriously research your friends' claims so that you can A) be there for them as support and a lending hand when they need it and B) so you don't get sucked in to their deceit if they're just pulling your leg (and everyone elses) so they can get attention. I know that sounds harsh, but there's really nothing quite like finding out you've been suckered by someone you care about and trust.