Hi Shpoog!
I am sorry to hear of your debacle and it is a problematic one. William gave great advice, and I agree, avoid surprises at all costs. People get very defensive when caught off guard. Try to have everything you want to say mapped out and review it once or twice before having the talk, and that way you don't leave anything out you want to say. It's easy to get sidetracked especially when a topic may become emotional.
Maybe as a lead in to the conversation mention that you have been reading a lot of health related studies lately (like the ones here for example), and it got you thinking about the health of everyone you care about.
I think the first step is to get her to commit, then once you do, figure out a plan that works best for her, and then figure out a good way to implement it. One step at a time as you don't want to overwhelm her.