nah thats not true! if she becomes a nice size then yes, she will get more attention, and she will never forget you for providing her an answer to her weight problem either...
dont get paraniod that she might leave you when shes thiner, that just shows lack of trust, and ime sure you do
trust her, dont you...
well anyway, she needs to be motivated, at the minuite shes probably rather lazy and will not like any exercise... so bring it up subtly, say:
"ime going to the gym, i cant wait, it feels so great just after you do a work out"
"hey, you can come with me , i could do with some company"
if saying that doesnt get a yes, then you keep on persisting, tell her all the great things to do , all the people to meet, and how great she will feel when she looses a couple of pounds..
if she still says no then you need to say it how it is...
if she is over weight, even only a bit, then it needs to be sorted, if she leaves it now then she will put on more and more weight and eventually die at the age of 50...