How do you convince a friend to stop overfeeding?

Perhaps you should also have a look at the owners of the dogs. Very often people who overfeed their dogs also overfeed themselves. If you really want to help your friends, and this is going to be a very tough and long-term project, you will have to start by showing them how to live a healthy lifestyle by eating a proper diet and having proper exercise. Only when the owners themselves do this would they be aware of the needs of their dogs with regards to food and exercise.

However if your friend's dog looks anything like this:

you may need to make a call to the nearest ASPCA or RSPCA office before it's too late.