Go massage her Schneider, but then pretend to get frustrated and say you can't hit the muscle tissue because there is too much fat and it's hurting your fingers. Tell her it's like trying to grap a penny at the bottom of a jello bowl.
That way you haven't actually told her she is fat, but you did provide enough evidence to insinuate that there is a weight problem.
OR
Go buy a jacket and insert lead weights, don't tell her about the weights, but wrap it up and say you bought a beautiful new jacket and you want her to try it on and wear it out for several days a week.