If she has already been cleared by a doctor for the dizziness as she says and they can find no cause for it, I may dig deeper into why her session the other day went fine, but others caused her to be dizzy for hours afterward? Did you change the program? Did she change a medication? Take it at a different time or different dose? Did she eat prior to your session? How soon to your session? A detailed outline of her day and what she did prior to the session and afterward, as well as what changes you made, may help clue both of you into what is causing her dizzy spells.
It could also be an inner ear issue. You mentioned that she is a swimmer. Water or inflamation in the ear can often cause balance issues. This might be what she is experiencing. Its quite common in swimmers.
Also, women experience menopausal symptoms for years, before menopause and after. It could be a temperature control issue brought on by changing hormones. Her body gets overheated, but due to hormonal changes, does not always signal to the body that its overheated, although it will act that way to protect itself. Much like hot flashes.
As you can see, there could be tons of issues. A detailed daily plan can help to give both you and her some insights. But like the others stated, if its a real concern of hers, even though she’s already been cleared to exercise, I’d have her follow up with a physician and push for an answer till she gets one.