>>21687553.5 is probably enough. Could go 5 if you intend to stay in the water for a full day. Thinner suits are a lot more comfortable, easier to put on and suffer less from buoyancy/compression issues. If she doesn't intend to do any serious diving (more than 10 m or so) compression doesn't matter much.
Haven't done any diving on the swedish west coast but done quite a bit in norway. Visibility is almost always better further out from the mainland and areas with some sort of current (chokes between island and such) will have a lot more sealife.
Eniro.se has excellent orthophotos and seacharts to scout out locations. I'd start by looking around the Koster islands. The closer to Skagerrak the better.
And most importantly, if she wont have anyone experienced with her make sure she doesn't do anything stupid, the sea doesn't fuck around. And boats, boats are the most dangerous things of all.
