resident dog depressed by arrival of new dog?

We went to the shelter and had a play date with a new pup so our over energenic dobe-boxer would have a play mate. She did very well with the dogs but the one she did best with was a lab-pitt mix. They were having tons of fun at the shelter and were excited to get home. They played when we go home and then that's when the problems started. They started fighting which my husband and I were mistaking for a decision of alpha. The resident dog had seem to won but as it turns out the more I look into it I think she was trying to challenge him and lost. Either way, my very happy very energetic resident dog has now fallen depressed. She hangs her head low and waits her turn for everything. She treads very lightly around the new dog and looks at us like she is very sad. Does anyone have any suggestions to make her a happy puppy again? We have considered taking the lab-pitt back to the shelter but we can't bring ourselves to do it.