condo unit becomes rooming house!

I am the president of a 6 unit association. Until last June, one unit was being used as a rental by its owner. Not a "normal" rental, however,--he was renting rooms out on a month to month basis to students (we are near a major university) and had walled up the dining room to create two more rooms. At one point, there were 7 people living in the 3 bedroom, 2 bath unit. We wrote and voted on rules and regulations that explicitly forbade this kind of rooming house arrangement and limited occupancy to no more than 3 adults. We also required one year leases. In September, the owner presented me with a one-year lease with the name of three women. It soon became clear, however, that this was a bogus lease. One of the tenants presented me with her real lease: a handwritten document that explicitly states that she is renting a specific room on a month to month basis.

What is the best way to pursue this? I've made a copy of the real lease. Should I speak with the owner, or just turn the matter over to our lawyer? In a small association like ours, friction is very difficult to deal with.