The Heritage Senior Center in Irving has a large pool with a ramp to enter/leave the pool. They also have water aerobics classes. I'd call them and see if she can join, even if she doesn't live in Irving. I know you don't have to be a senior to participate in the water aerobics (although I believe the "senior" age minimum is 50, so she might qualify anyway). You might also try community colleges / rec centers in her area and see if they have pools with lift systems or ramps.
Also, I think there's a rehab center/pool near Bachman Lake in Dallas (my dad qualified to go there because of his diabetes and a doctor's prescription). Don't know if they have a ramp/lift, but I would imagine they do.