If Tristan is shocked by this reveal, this explanation for Constance's brightened mood, he keeps it from his face. It is not his place to ask if this is too much, or too little - it is his place to trust that Constance has made the correct decision, and to support her if she needs it. To stand aside if she doesn't. So he stands aside. 

A good hunter knows when it is time to watch and to wait.