Good luck! I'm excited for you!
Just be agreeable with the owner and listen to what she says.
Once the dog is settled at your place you can change his diet and go ahead with whatever improvements you feel he needs in his routine.
I feed my Mini-D a cup a day of high quality dry food (also for her teeth, which are prone to tartar buildup) and count in the treats she gets into her total caloric intake to keep her weight under control...it's hard with the little dogs...
55 g Goldfish Tank - 3 Fancies, 2 Comets
25 g Tropical Tank - Celestial Pearl Danio/Mixed