This is not legal advice:

It is not illegal to fire someone for not being at work- regardless of the situation, unless the firing is discriminatory in nature. The boss isn't firing/threatening to fire Kelly for her disease (and, Kelly, does this qualify under the ADA or not?), but, for her absences.

She would seek an employment lawyer, not a labor lawyer (unless her workplace is unionized).

She probably doesn't qualify for FMLA, as she works for a small company, and the FMLA doesn't apply.

Disability is probably the smartest move to make.