What a fabulous location for an experience in Paris! It's a large and stylish place very near two Metro stops and multiple cafes. Be sure that the couple using the "upstairs" room is ready for a climb as those few stairs are a bit narrow! However the other room options are much more accessible (ie no stairs!).
Finally, just be sure your host explains how to use that key! The doors, like the rest of Paris, are a bit older than we what we get in the States and require a little bit of love to operate.