The size of the bike is definitely important, but reach will be more important than inseam. If you say she tried this bike (but a newer model), do you know what size she rode then?

Honestly, if I were her, I'd appreciate your efforts but would probably want some input in the choice of the bike (better than a surprise gone wrong, hmm?). Maybe call up the person selling it and bring her to try the bike, but just keep the price secret. It might hurt the surprise factor a little but it's easier than trying to re-sell the bike on CL next week b/c it didn't fit right.