Hand foot and mouth disease can be pretty miserable---so I wouldn't think she'd want to swim if she's in the active phase of the disease with fevers and blisters. Transmission can come from the saliva for upwards of a 7-10 days and from feces for much longer (definitely no pooping in the pool!) If she has been afebrile for 48+ hours and has healing blisters she's probably through the worst of it. I think it really comes down to your comfort level and accepting that you could potentially catch it. Keeping the pool at the high end of regular chlorine levels when she's in it and then superchlorinating when she's headed home is likely fine. You are probably more at risk of getting the disease yourself from cuddles/smooches and sharing utensils or drinking glasses. This virus goes like wildfire through little kids (think daycares/schools/camps) but the transmission to adults is less common.
From the CDC website:
--It is possible to get infected with the viruses that cause hand, foot, and mouth disease if you swallow recreational water, such as water in swimming pools. However, this is not very common. This is more likely to happen if the water becomes contaminated with feces from a person who has hand, foot, and mouth disease and is not properly treated with chlorine. (**I didn't see specific chlorination recommendations)
Hand, foot, and mouth disease is spread from person to person through coughing and sneezing, or contact with blister fluid or feces (stool) of an infected person. You can lower your risk of being infected by
-Washing your hands often with soap and water
-Disinfecting dirty surfaces and soiled items
-Avoiding close contact such as kissing, hugging, or sharing eating utensils or cups with infected people
Hope the kiddo is feeling better for her visit---whether she uses the pool or not!