Always have water boiling, ha ha, that was good. My DH and I went 3 years ago when I was also 6 months pregnant. Good advice of course, is to let her sit fairly often and take a lot of breaks.
One thing though, we went in April just before it got really hot. You are definitely going during a hot time (and crowded time). So, make sure that she doesn't push herself too much. Sit her down somewhere in the shade when you are going in the stores or on rides that pregnant women cannot go on.
But, the most most most important thing is, make sure she drinks A LOT of water. That is one of the most important things. Dehydration is so dangerous for pregnant women and she'll really need it then with the heat. Make sure she gets her own copy of the park map to plot out all of the bathrooms
I also brought a map of Orlando and plotted out where the nearest hospital was just in case. (I felt more at ease knowing where it was).
And just keep in mind all of the common sense things, comfortable shoes, sun visor, sunscreen, etc.
18 more days...