I was lucky enough to shed my wig by early July (in Michigan), so the heat was not an issue. By June, I had enough hair to wear caps for outside and yard work, so I never had to wear the wig outside. I would recommend going to a wig shop, preferably one that specializes in treating people going through chemo. In my opinion, it is better to be able to see them, feel them, examine the stitching, etc and especially try it on. However, a lot of people have gone on line and ordered them with great success too.
She may find that she prefers caps or scarves or bandanas for summer time, some people do both.