Over here Australia women of all shapes and sizes use wetsuits, farmer Jon style two piece are my favourite.
The rubber they are made from makes a huge difference to the comfort of the suit and thicker doesn't mean warmer, it often means almost impossible to get on and off.
I bought my wife and eldest daughter cressi 7.5 mm which are very hard to get into and pretty hard to move in once they are on. These suits are for our winter where the sea temp drops to 13'c. Unfortunately they were put off by all the effort getting in and out of the suits so soon gave up on the whole idea.
Since then we have all moved over to open sell Yamamoto rubber suits, this rubber is about as good as it gets for stretch and comfort. The suits need lubricant inside to slip on, which is extremely easy and almost feels like you are naked they are so comfortable. The fact these suits stretch so well almost no water penetrates them meaning they slip off easy due to the lubricant and a 3mm suit actually keeps us warmer for longer than the 7.5 cressi and mares suits we own. We buy our suits direct from Mako Wetsuits-Hoods-Gloves-Socks
By the way is she intending getting in and out of the boat to dive or snorkel ? as this can be quite a challenge for the less fit.