There's a few grain-free options that are pretty low in fat/calories. California Nautral carries four grainfree varieties now (chicken, salmon, kangaroo & lamb).
Wellness Core Reduced Fat also offers a nice formula for weight loss. A lot of the Go! and Now! formulas are made pretty simple... a bit lower in protein/calories/fat.
I would just feed a bit less, exercise more.
How much do you think they have to lose?