If this friend has been told by a doctor that they should do this, and they signed up WITH YOU for this class, they know what the class requires (i.e. a swimsuit). I vote that you forget the swimsuit one day. "But why don't you have 'my' swimsuit??" "Oh, another friend borrowed it and hasn't returned it yet. Why don't you have your own ready, you knew we were going swimming today." If she starts sputtering about how she expected you to let her borrow 'her' suit, tell her once again that it is a suit that you allow all friends to borrow, it is not hers alone, and if she wants one JUST for herself - bring one from home. Ask her if she even has a suit, period. That should give you some insight.
Sure, you can buy a suit at Walmart (in the right season, or even online) for fairly cheap. You get what you pay for, though; a pricier suit bought from swimoutlet.com lasted me much longer than a Walmart suit and fit me even better (granted I have an odd body shape). It could be she can't afford to buy one of her own. If she can afford it, but just won't for some reason, tell her point blank she needs to buy her own and stop relying on you to provide something that is not hers. If she shows up without a suit, tell her "oh well, you can sit on the bench or here's a great book I got last week you can read it in the car while I take my class." You signed up for it. You're paying for the membership, so you're paying to be there. I would make a friend sit and watch if they "forgot" their suit.