Come to think, if she brings up the cost of the taxi home, you could point out to your friend that her partner should have thought of that when he arranged a 6 a.m. arrival. If he booked the earlier flight because it was a lot cheaper, they'll still come out ahead. If he chose it thinking "oh, this one will save thirty dollars," maybe next time he'll take airport transport costs into account.
As a side note, while I sometimes select flights for myself and my spouse when we're traveling, I don't do it without getting his input: I tend to look at things, and say "the options that look reasonable to me are these, what do you think?" I wouldn't book a red-eye if we could avoid it, for him, me, or both of us, because I know we don't like those.