since each person has a different definition of modest, I think it is hard to assess each person's modesty during the summer.
for myself, I wear what is not uncomfortable. that means shirts with at least cap-length sleeves, bermuda/knee-length shorts (either fitted or wide) and wide skirts.
Bill: has your wife tried looking at the bermuda-length shorts? there are some really cute ones with wide enough legs to be cool even in the extreme heat. Old Navy tends to be pretty modest with most shorts/pants and they have really, really good sales so you don't have to pay an arm and a leg for a decent pair of shorts.
http://www.oldnavy.com/browse/category.do?cid=26197 (my link thing isn't working for some reason) is the sale section for bottoms. she can browse around and decide if she even wants to bother going to the store or not.