Maybe for longer rides that she can't complete yet, you two start early and then meet her for the last third or so? Hopefully then your being more tired combined with her being fresh would help keep you guys together.

I think her husband needs to understand that if she insists on riding the comfort bike and not training during the week she is somewhat resigning herself to not keeping up. She may never be as fast or strong as you guys, but she's not even meeting you halfway.