FWIW, I've been a T1 for 14 years. Insulin dependent as pills don't do anything for me since I have a non-functioning pancreas.
Obviously, in a hot environment everyone needs a lot of water to keep hydrated, diabetic or otherwise.
I let my blood sugar ride a little higher whenever I'm going to be in a high exertion and high stress situation. I'd recommend looking into slow digesting carbs that will keep things a bit more stable. (Book: Glycemic Revolution). Test more often, as annoying as that is, and keep some high sugar, quick digesting stuff handy. I usually keep some small cans of non-diet Mountain Dew handy to bump things up if needed. It seems to work fast in me, at least.
She should get a feel as to what is needed and remember, everyone is different. For example, sweating raises my blood sugar when the Doc says working out should lower it.
PM if you want.
John