Man, after reading these replies I feel like a reptile!!!
Maybe I should even consider some suits. I do know 80% of the heat lost is through your head, so maybe I'll just wear a hood
In all honesty, I think I will have my wife wear one of my old 3 mil wet suits in the warm temp and see how she does. I used to wear my 3 mil for 58 degree water while surfing and I would get hot. I know its not a similar comparison, so we'll see how she does.
On a side note, not sure if she thinks its dorky, but I think she wants to wear a shorty for appearance sake. Women, always fashion over function ;-)
There are a few things to consider...
If you move around a lot you are going to generate heat. Last dive trip I was on I hardly moved from the waist up and only occasionally from the waist down. In 75-80F water I was cold wearing a full 3mm.
Additionally, what kind of wetsuit? How deep do you go? Some wetsuits compress more than others. If you are going deep then a hyperstretch will probably compress a lot more than other types of wetsuits.
I think a good 3mm will not hurt. She might even be okay with a 5mm or a 5mm shorty.
By the way, you are probably a lot more active when surfing then diving and without the depth there would be no compression on the wetsuit.