A thicker wetsuit isn’t necessarily going to what she is looking for. Yes more neoprene will be warmer, but a perfectly fitting wetsuit will give her warmth and flexibility. I’d recommend trying some suits on and finding the one that fits her the best.
For the most part the thickest panel you will find is 5mm, as that is the maximum legal thickness with most governing body’s. That 5mm panel will likely only go down the front of the wetsuit. A 3-4mm panel is usually the max thickness down the back of the suit, and anywhere from 1-3mm in the arms and shoulders. The thickness’s and flexibility of every suit will vary depending on fit, thickness of neoprene, suit construction, and flexibility of the neoprene.
Ultimately fit is should be the number one concern. A proper fitting wetsuit will perform at a higher level and help to keep her warmer.
Jake