I had something along the same vein happen to me once. All I can say is this. Your other friends know what happened. They could tell you exactly what went down as everyone was walking to the bathroom, what happened at the bathroom, what Steve was acting like, what your wife was acting like, etc. And if they knew they were in the bathroom together, why would they leave them there before returning to the campsite? Most likely they aren't going to want to be completely frank with you because they'll feel like they shouldn't get in the middle or it isn't their business. But if they're real friends, it is their business to tell you.If it was me, these two things would make me suspicious.
A few minutes later, they came walking back to the site together. George asked them what was going on and Steve just laughed.
She remebers things from the night, but does not remember going into the stall with the guy, nor does she remember anything happening. She kind of freaked out about this, and I certainly wish I'd have listned to George and walked to the bathroom to check on them.
I don't care how drunk you are- people don't just forget about things like this. You may forget minor details, but come on. It sounds more like two people who are being deliberately vague with you. Sorry to be the voice of negativity and I hope for your peace of mind that nothing happened. But I'd say you have every right to be suspicious and pissed off.