I don't see a big difference if she pays low fares. She could finish AA's Platinum challenge on her last flight, too late to do her any good as far as this assignment goes and of no use if she isn't going to fly AA in the future. She could get Gold sooner, which would be of at least some value for early boarding, being able to select exit row seats and such - as well as getting her 2,100+ bonus miles on her last five round trips. If she pays higher fares she could become Platinum in time for it to be useful.

UA is also not so great: she couldn't select E+ unless she pays a higher fare, since she won't have any status with them. The 17,000+ miles she'll earn this way won't get her any. E+ is a plus if she pays a high enough fare to sit there without status, though.

In my experience with both (UA 536 ORD-BOS most recently last week, lots of flights on AA as well), there isn't a lot of difference in service. In both cases most of the people she'll deal with will be competent professionals with a good attitude. A few won't. It's the luck of the draw on any given day.

To me, it's a toss-up. I'd base the choice on which airline looks better for whatever flying she expects to do down the road. As you move up the pecking order, differences start to show more.