For triathlons, the rules require me to wear a wet suit. It keeps my legs high in the water (which causes me problems I won’t describe here). A wet suit top could add buoyancy to her upper body. Likewise, scuba divers have gear (usually weights but can be air in a bladder) to create neutral buoyancy in the water. Go ask a scuba instructor, at a dive shop, or https://www.diveheart.org/ if they have ideas. I have dived with Dive Heart, they have experts experienced in working with people with various physical deficits. Dive heart does require medical clearance to participate in their events, but they might do a pool side consult that wouldn’t require a medical clearance, all you can do is ask.
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