I don't see any way for her to avoid airline requirements to check in (at a massive airport with some famously long walking distances) three hours before departure time. I suggest she look online a few times now, for some common departure gates used by her future flight home.
As others have mentioned in unrelated discussions here, you have to have a fallback plan in case of transportation difficulties. We were staying at the end of the Kronberg (Frankfurt) S-Bahn line, with non-changeable train tickets to Berlin, in a peak travel month. Our planned S-Bahn train had a passed-out drunk on the floor, and even half-an-hour after the police arrived, there was no sign of the train departing. We called a taxi to the HBF, so as not to miss the train.
I've probably been to Frankfurt three times, for various reasons. It has some excellent attractions, but I would comment that it lends itself less to random walking around than do some "older" European cities. I would suggest that she make a plan for her limited time, in order to make the most of it.