We tried not being in a party but she just got no EXP (nor challenge log progress) if I hit the monster. Heals did not cause EXP loss, but they were not really required either so it was very boring for me.

While doing the FATEs I noticed that Lv16 monsters still gave me like 1 EXP per kill, so perhaps that would be the limit where she'd also get credit while in party with me.