This is really not such a difficult situation. As always, people suggest what they have or like...while the focus has to be how can the OP get the most out of his ONLY lens in the beginning.
I suggested the 50mm for him, even though my personal preference is for the 35mm as a standard which will NOT suit his needs as the only lens. The 35mm offerings generate varying levels of distortion if he has to get a tighter shot by getting closer. 85mm and 100mm are too tight in most rooms, and certainly not ideal for indoor groups.
50mm is ideal as you can walk back and acheive near 35mm FOV or walk closer and get tighter shots that still look normal.
50mm allows him to zoom with his feet and comes in a wide price range that can be as cheap or as expensive as he wishes. It is a no brainer