I won't go for the male/female angle which so many have posted about. Instead its the FF vs non-FF that struck me.
If only one of them were going to get the F seat, then it should go to the non-FF. They would consider it an absolute treat to be up there and therefore appreciate it all the more, it's not something they experience all the time. Personally, I don't see that "first class" on the bp as that big a deal anymore, as I get the opportunity so often. I'm fine when I'm in Y, but prefer the upgrade when it clears but its not that big a deal overall (domestic, folks - for intl, it matters). OTOH, someone who has never flown in F, even domestic F, would have a blast, it'd be a treat, a gift and an appropriate gesture from one who has already had the experience.
The other part is that while these are two people flying together, it seems the FF'er dropped their non-FF friend into the back way too quick and grabbed the upgrade. If the FF'er has an issue travelling with the GF now, it'll only get worse in the future. From the OP, it doesn't seem like there was any hesitation that they'd be taking the upgrade and screw the other person, not even stopping to think about refusing the upgrade, giving it to someone else, etc. That lack of consideration would be raising some serious flags in my mind.