Ugh Mono. I dont wish that on anyone. I got it about two years ago, 28 at the time, and it was the worst thing I ever had. I was sicker than I have ever been in my life, was weak, had no energy, couldn't walk without help, felt like my body was dying, slept like 20 hours a day, and was out of work for almost 3 months. The doctor did tell me that the older you are the worst it is. But I wouldn't even want a fraction of it.
Get your DDs results and find out what she has for sure. There really isn't much you can do for Mono besides rest, but you want to make sure she gets the right diagnosis. I was misdiagnosed in the begining and the meds they gave me worked against me and made things worst. Took about a week, 2 ER visits and 3 doctor visits to figure out that I had Mono. Plus as a bonus I had HepA at the same time. How lucky was I? Yep, I had the cooties!
Mono is not fun. Maybe your DD just has a nasty bug instead. Hopefully you will find out what she has and she can get over it quickly. Good luck!