Sounds like you like him so you feel bad about it.
Bit of deflection?
If you didn't like him at all, it would have been awwe thanks I love you too man/friend and he really would have been like
Either way he might have just been tipsy and really does have love for you but wouldn't act on it.
I don't see the point and running and telling his girl that he said that while drinking if he never acts out, pursues or makes you feel uncomfortable.... It's basically ruining a relationship or planting that seed of doubt.
unless subconsciously you want them to break up so maybe you and him can have a shot or there's a bit of envy or pride.
Sounds like there's some tension there. Believe me when you aren't attracted to someone on some level, you don't say that and you don't respond like that either...
Anyway it sounds harmless as long as no ones acting on anything, but its a prime reason why men and women can't all be friends.