Should I get my Northern Green frog a friend?

If the 55 hex is taller than it is wide, I would reconsider, since these frogs do not normally climb. Most frogs are completely happy on there own, and another frog means less territory and less food, so most frogs arent to welcoming to each other except when breeding. It is completely fine to add another frog, but don't be surprised if they fight, but they probably wont. Also make sure the new addition is the same size as your current frog.