Went to play Bag End, got her to visit, they talked a bit more. Now they're friends again, finally. He even has a smiley-symbol for her, which he didn't before. Then Frodo and Sam kissed in the hallway. -sigh- Rosie came running out of the kitchen and slapped Frodo.
Then she left. His scores for her are just as high, but he lost the smiley-symbol, so I guess she's no longer a friend. And I'm not sure if it's connected or not, but Frodo wouldn't "play in bed" that night -- Sam had to start it. (Though at least the interaction afterwards was Sam cheering Frodo up, rather than Frodo booing or slapping him, as in the past!)
I don't know, I swear I've heard from people that Sims can fall in love with more than one person at once, but it certainly doesn't look likely here!