Were there 2 witnesses to see they had been in the same house overnight, or did your friend admit she was a roommate?
I know 3 cases where the wife moved out and was living with her boyfriend while still married, and the husbands had to camp out overnight outside the house and take pictures that the boyfriend's car was there. They also had 2 people in the car with them to have 2 witnesses. The elders wouldn't go on heresay, even though one of the women was frequently seen around town with her boyfriend.
In another case, my former friend left her husband and lived with a few boyfriends for a couple years. No one had "proof" ( videotaping or taking pictures that the guy's car was there overnight) so nothing happened. When she moved back into our territory with her boyfriend, the elders visited her a few times and she said the guy was just a roommate. She was finally DF'd a few weeks later after she married the guy.
Your friend could appeal to the CO and say there aren't 2 witnesses, but it may not be enough. I'm sorry to hear about your friend, and I hope he's able to pick up the pieces. All 3 of the 4 guys I know are remarried, some not scripturally, but they are all happy and have better marriages then before.