Welcome! I don't know if this helps, and it is clear the rules are not standardized, but I'll relate how a similar situation was handled, though the wife's behavior instigated it.
A very attractive wife was married to an unbeliever but found a man whom she met in field service whom she decided was the ideal mate and decided she wanted to marry him, which she eventually did. But she knew her husband well and his infidelities so withheld sexual favors from him, knowing quite well he'd likely get it elsewhere. So she had a close friend in the truth follow him. She had the same task as you to "prove" adultery. But the elders told her that if he was alone with this woman and in her house and the "lights went out" then she could get a divorce. She didn't have to prove they had sex, only the reasonable presumption. Now they probably did have sex, but all that seemed to be required by the elders was his being alone with her and the light going out. He was devastated to have been found out and that his wife was divorcing him. She got the divorce and promptly married her Bible Study once he got baptized.
Oh, and in another case, where an elder was guilty of fondling only several different women, the elder was disfellowshipped. But no technical fornication took place. The wife, though claimed he betrayed her in his heart. The elders told her even though he was disfellowshipped, though, since there was no formal "adultery" she could not divorce him. She, however, basically told the elders to take a hike and divorced him anyway. There was little the elders could do. And later she married another brother and their marriage was accepted, whatever.
So it just depends. Like someone said, hang around and the horror stories will amaze you!
LS