<human>: Summarize the given text in a few sentences.

Have a good friend who is generally considered the nicest guy around, have known him for years. He's single.

Was in town with myself, gf, friend, and a few others, including my gf's sister, who had to leave early.  My friend, who unfortunately isn't always the clearest, most self-aware when drunk, asked my gf why her sister was leaving, as he thought she was the 'cool one' (among some other comments).

This hit a sore point she's had for quite some time, so now unfortunately she and her sister have decided it was very inconsiderate/offensive (which it was, it's just totally out of character for him and he'd be mortified to know he'd upset her).

How do I broach this with him, with out upsetting him, but still doing right by my gf?

I assume I should be the one to bring it up with him, but don't want to make a rift, and don't want to upset either party further than has already happened.
<bot>: How to explain to nice-guy friend that he's (I believe accidentally) really upset my gf with a back-handed insult?