The responsibility you are taking on is tremendous. You are being very generous with such an offer and good gesture.
If I was in this situation I would have a long talk w/ your friends about your concern over their puppy. Possibly even having them write down their vet's #, how much food to feed, what the pups daily routine is, a copy of the shot record, etc. Any information on him will help just in case an emergency arrises and you do need to go to the vet, her vet or yours.
Changing his food is not a good idea in my opinion. It could possibly cause harm/stress on this little guy.
Also, taking this little guy to the vet needs to be brought up w/ your friends. This is THEIR baby and might become quite offended w/ you taking them to your vet behind their back.
If you gave them YOUR fur baby and they switched the food and took your fur baby to the vet w/ out your consent this could get really touchy and cause ill feelings.
I just feel that a long chat w/ your friends before hand would be so very helpful. This way, everybody is on the same page of each others feelings, what care is to be expected, etc and nobody should get hurt/angry or have any questions.
With open communication your friends will see that you just want whats best for their pup and all of yours friendship.
I hope it all works out.
