You don’t need to sleep with him at the party! If you get his approval high enough, he’ll proposition you on his own at some point during act 1 when you talk to him. So your friend just needs to keep getting his approval up and it should trigger eventually, though it should probably be done before moving on to act 2. The romance doesn’t progress any differently than it would have if she had slept with him at the party.
Killing the Gur monster hunter in the swamp with him in your party
will net you a lot of approval points.