my opinion is that we don't know enough about the situation and everything is based on assumptions. legally, yes it's wrong because you can't legally consent.
but you need to ask motives and desires and that's a key role in investigating and understanding the scenario without jumping to conclusions. does he do this often, does she do this often, did they want it, all of those are simply left out of the story and we don't know. if everyone in the world lived their lives according to laws, then of course it's bad, but people don't live their lives thinking about doing only legal things for their entire existence.
but it seems you've made up your mind and are insistent on losing this friend over something that didn't directly involve you, that's the only real fact here
Disclaimer: There's a good chance the above post could be sarcasm.
Die-hard Oakland A's fan --- Keep the A's in Oakland!