Is my husband's frustration within normal limits?

We're watching someone's dog this weekend and I'll admit, he's really hard to handle. He poops on the floor even after he's just gone outside. However, my husband's reaction to it, according to me, is way over the top. He yells and screams and curses at the dog and mutters for the next half hour. The dog is clearly a little wary of my husband now, and they're usually best buddies. And then he stays so mad at the dog that he doesn't show him any affection at all, which is the whole reason his owner wanted to leave him with us instead of boarding him. He even yells at the dog about it during times the dog is being calm. My husband is angry with me for being angry with him about it. However, I think he's bordering on cruelty to the dog and it makes me not want to be close to him. My husband says he just needs somebody to listen and not be mad so that he can vent his frustration. What should I do?