I'm in a very similar situation. My wife used to cycle but has recently put on weight and she has become self-conscious on a bike. She's gone from a size 10 to a size 14 over the years and it bothers her much more than it should. She also has had trouble with her knees and cycling seems to aggravate the problem.
So it's me on my own from now on. The earlier post which stated that being with just one companion on a tour is easier than being with a group is spot-on; fewer people to consider.
It can be annoying when everyone assumes there is an ulterior motive when cycling with a friend who just happens to be a member of the opposite sex. I would gladly tour with another female, but my wife would never agree to it.
I too prefer the company of women as there is no macho posturing or competitiveness. That's the last thing you need on a tour.
Size 14 Isn't that about average these days? She shouldn't need to feel self-conscious ...
You say cycling causes her knee problems ... is the bike set up properly for her? (Apologies if you are an expert - I just thought I'd mention the obvious

T