Dog gained 15lb in one year

I am asking question on behalf of my friend who is very worried about her 7 yr old male golden retriever. She is a wonderful, caring pet owner, who, for the first 5 years of her doggie's life fed him food made from scratch, consisting of raw chicken, liver and other organ meats, eggs, and raw veggies, with no additives or preservatives. She prepared this diet at the recommendation of the vet/breeder who sold her the dog. On year 6 she acquired two more retrievers and decided to feed them store bought dog food (science diet) because it was too labor intensive to prepare the chicken/veggie/egg meal. After a year of Science Diet she has gone back to the chichen/veggie/egg diet regime and her 7 yr old dog has gained 15 pounds in less than 1 yr. He should be about 75-80 lb and instead he is close to 95 lb. The first 5 yrs weight was in line, the year eating science diet weight was in line, now on yr 7 he has balooned.

Any comments on what might have caused this sudden gain? She has lost touch with orig. vet and new vet not much help. She notices dog is on the lethargic side these days.

thanks for your help