I don't think a doctor telling her would be any different. I know for me it wasn't... I had a doctor who was VERY blunt and even though in my head I knew she was 100% correct, hearing it from her made me less interested in changing for some reason... Hearing it from my friends, family and husband didn't help either... I had to get to the point where something kind of clicked inside me...
I agree that getting healthier is kind of contagious... my mom started weight watchers late last year and has been losing about 1.5 lbs every week consistenly since she started. She has been walking every night and eating healthier. I think shes about 3 months in and has had a loss every single week at her weigh in meeting. She now weighs about 25 lbs less than I do... I have to say, that was in part a motivator for me... we live about 3 hours apart so clearly can't walk together etc.. but its kind of fun to compare how much (walking etc) each of us did that weekend etc...
Also, in my own home, I'm cooking healthier and now my husband and step daughter are in turn eating healthier and now I find them WANTING to eat healthier and kind of looking forward to healthier meals etc... so its kind of a circle... none of us were eating healthy because I (the primary shopper/cooker) was not preparing healthy meals, and now that I am, everyone is benefitting and its catching on... my husband has started going to the gym and my step daughter signed up with a friend of hers to go to a trainer once a week and on the other days is walking etc... then in turn I see them making small progress and so in turn I want to make progress, kind of like peer pressure a little bit I guess....
Its harder since you live apart... but there are still ways you can encourage her without being direct... sharing healthier recipes without being overly obvious etc... maybe you can see if there are activities in her area.. say.. an arts festival or something like that where you get out and walk around the booths etc and say ooh that looks like fun, why dont you and dad go... or if your dad is on board maybe he can say hey hon I'm gonna go for a walk to the end of the block and back will you come and keep me company? Heck, even a walk around the mall... anything to get her up and moving a little bit at a time and hopefully it snowballs....

