how do you know when your dog has had enough or too much exercise?

[Brandi]

i think my boyfriend over exercises our dog. when he comes home hes panting wildly and drinks a lot of water then throws up all over the house. he also has the diarrhea for about the first hour after they get home. i know that some of it is normal but i need some back up or advice about how to tell when hes had enough because i cant get it through my boyfriends head that a dog can be over exercised. my dog is half saint bernard the other half is german shepard and lab, thanks!