I have not read all of the other replies but this is my advice..
my mom never cared about my weight at all. I have been over weight since elementary school.. that is NOT ok! If she is coming to you for help.. and she is if she is telling you she feels over weight.. help her out! Go on walks together, make better choices of the foods that she is eating. Show her that healthy eating can be fun and taste good. I have a book called eat this not that.. I am pretty sure they have a kids book too. That may help.. Good luck. I hope nothing I said sounds harsh. Don't get upset about it.. let her asking you for help encourage you!
Good luck!