AUH is a small airport. An hour and a half, especially on the same airline, won't be a problem.

She also won't have to go through customs, etc.

That said, she could take the layover to break up the trip and enjoy a nice half-day in Abu Dhabi (about 40-minute drive from the airport) or Dubai (about 75-minute drive from the airport). There's not a lot to do in Abu Dhabi, but, with that much time, she could take a stroll on the corniche, have a nice Arabic lunch (unless this is during Ramadan, which runs through the next month or so), go the beach, or at least kill time at the Abu Dhabi mall.