Renting an AR is a good idea! Many first time AR shooters are uncomfortable with the proper forward positioning of the head in relation to the stock, trying to keep the head back away from the charging handle. That rearward position makes sight alignment difficult. She needs to know that the recoil won't hurt her, and that her nose should be close to the charging handle. Some AR shooters actually put their nose touching the charging handle and don't get "punched in the nose". YMMV.
I am a ‘nose to the charging handle’ type of guy. I haven’t had any trouble with the recoil punching me in the face and I get the exact same cheek weld and sight picture every time, iron sights or optics. It also helps maintain the proper eye distance of your scope doesn’t have 3 or 4 inches of eye relief to play with.
The only con is I can get a snort full of hot gas if I’m shooting fast.