Mostly, Fred was comfortable with it by not thinking too much about what it really entailed. It was a coping mechanism, but he wasn't knocking it, since it seemed to be working. And he assumed she could see George in her future daughter, too, but he also wasn't going to bring that up, especially if she wasn't.
"Maybe I'll just host a party. 'All Weasleys and related who haven't met Fred, please show up at such and such time. And the rest of you, too; everyone loves a party,'" Fred joked, grinning at Angelina. It wasn't a bad idea, though. Get it all over with at once.