What should I do if a dog in my care attacked my dog - leaving bite wounds?

I have never had anything like this happen before! I was cleaning up the kitchen and I gave each dog a treat then turned to put something in the sink, so I am not totally sure how it started, but I think the other dog dropped part of his treat and my dog then got too close.

This major fight erupted. I instinctively grabbed my dog's collar and pulled him back - but this other dog kept viciously attacking him! He was biting my dogs neck and leg. Luckily my boyfriend heard the commotion and came running in to help separate the dogs. This dog did not stop until my boyfriend had him completely immobilized in a bear hug - then my boyfriend put that dog in the bathroom while we checked our dog.

My dog is limping and has bite wounds. It was so bad that we had our dog sitter come get our dog to keep him until this other dog is picked up tomorrow!!!

So, I am not sure what to do. I need to let the owner know what's going on - but not sure how to handle that. I also might also need to have my dog checked out, but right now getting him to a safe place was our first priority. Also, when we put the other dog in the bathroom, he pulled a panel off the tub and somehow got a little cut on the top of his head - so I am worried the owner will think my dog did that.