There's this Neil Simon play that's also a movie (maybe Lost in Yonkers) where near the end, a teenage guy is keeping his mother company while his father's been away. She's reminiscing about old days and is finally having a happy moment thinking about old times. During that, she plays music the she and her son's father used to dance to. She shows him how to dance to it with her and it's a really sweet scene between them.

The teenage guy is narrating and he has a really good thought about it where he talks about it being a really important moment with his mother, but the dance itself was something that should have been for his father. It's not because of the ritual, but because of the real love in that moment was something that should have been channeled into his parents weakening relationship instead, even though it wasn't romantic love.

That's always stuck with me as a way of viewing love and energy and where we put it. It's not that it's wrong to have those moments with others, but that if we're working on a partnership with someone, that love should be focused on that person. We sometimes think of only romantic love as off limits, but other kinds of intimacy can kind of spread us thin and prevent that deepening with the one we're choosing to build more with.

Anyway, I think you're on the right page for this guy and are a good friend for thinking about what's best for him and his girlfriend. This is how I'd end up describing it. It's not that the intimacy between each other is wrong, but that a lot of that should now be directed at her. If he soothes his emotional needs with you, he won't learn to take those to her and grow with her as much.