Pay more attention to your friend. Overweight is one of the biggest health problems for dogs in the US. Treats are over used in training too. I seldom use treats. In special cases where I use treats, I usually just use a little of the dogs kibble. I will never forget the one lady at the grocery store that went on and on about what all her Springers would do for a Cheerio. The dog guide school teaches us to take a regular dog biscuit and break it down into little pieces half the size of the end of your finger.
Over the years, I have seen many questions from people that have taken their dog to obedience class, and still have little control over it. The dogs see all the people and dogs in the household as a pack with each having their own rank in the pack and a top dog. Life is much easier if the 2 legged pack members outrank the 4 legged ones. You can learn to play the role of top dog by reading some books or going to a good obedience class. A good obedience class or book is about you being top dog, not about rewarding standard commands with a treat. Start at
Raising Your Dog with the Monks of New Skete.
If your dog is losing interest in its treats, it may be over fed. There are some dogs that just are not as food motivated as others. There are even Labs that aren't. Here is a good guide to feeding your dog.
September 07, link to chart replaced with
LongLiveYourDog.com - Life Span Study - Rate Your Dog