I think a lot of people gain weight in their 20s when they go to college ( the 'Freshman 15').When I was that age, I think it would have been pretty mortifying to have my dad tell me he thought I was overweight, even if he had good intentions! Girls have very sensitive self-esteem at that age. At 5'8", and the weight range you suggested, she could be a little overweight OR she could be a very physically active, perfectly healthy girl who is just tall and big-boned!
If there are hereditary health issues, what are they? I think you have to factor in: does the daughter smoke? Is she physically active? Does she work out, run, swim, play volleyball, etc? or does she sit around all the time eating junk food and watching TV? Those things make a huge difference. If she sees a doctor for regular checkups, she is probably well aware of any weight issues.
I just think this is a very sensitive subject, especially with young women, so be very careful how you approach it. It could result in really hurt feelings between father and daughter if not handled the right way.
If he has health concerns, maybe a better way to bring it up would be, "Now that you are in your 20s, I wanted to let you know that our family is prone to high blood pressure, diabetes, etc. when we get into our 30s. It's never too soon to take care of yourself." or something like that, rather than " You look like you've gained weight and we're prone to health conditions."
It's a touchy subject- you're a good stepmom for trying to help out with this! Good luck!