I always felt he always had feelings for her, but knew he was "friend-zoned" and accepted that their relationship would never go further. If you remember, there's a scene where he writes his feelings for her in her yearbook when they were in HS, and then gets ridiculed at a keg party by all the "cool kids" who found Jaime's yearbook and his confession.
So, when they meet up again later, Chris initially does just want to sleep with her, but then decided her friendship was more important than a conquest. There's a scene where he intends to tell Jaime how he feels at her family Christmas party, but gets blocked by Dinkleman.
So, to answer your question, it's before they spend the night together that his true intentions are known to the audience, as that was the night following the Christmas party. But it's while they're in bed that Jaime finally understands it.