Mononucleosis is contagious. I can absolutely understand why the company or the union might want her to take the time off. Especially working in a retail setting.

If she's union, she probably has some kind of sick pay or disability pay that would kick in. I'd be going that route and give her the time to rest and possibly not infect anyone else. While getting paid.

Just imagine if she sneezes in the vicinity of another employee or customer, or licking her fingers to grab bags or papers, the panic that can ensue. Do you let everyone know? Or do you not, and take the chance another employee might accidentally take a sip from her straw and oops...wrong drink, that one wasn't mine ha ha...

I can totally understand why the company would rather she just stayed home.

And whatever the ADA states, I doubt it has anything to do with contagious, infectious disabilities when you are working in a very public setting like that.