First of all ask yourselves why do you have such a restrictions? These are not easy times and if an owner has a roommate who pays it literally means the owner then does not ho into default . It is hard to believe that any Brd can be so selfish and intrusive to decide if an owner can have someone live in their apartment or not. 
Wake up to reality. It is not a crime to have a room-mate.

First, you should check your documents. It may state that a homeowner cannot lease less than a whole unit and may define a period of time, for example; that the lease may not be less than six months or a year. 
 
Second, check state law. It also may have requirements for condominiums and leasing, though they are usually very general.  
 
Third, then check city ordinaces. 
 
As a group, you may also want to seek legal counsel. You also may want to tell him that any living situation in which he resides at the unit and has a roommate, friend or relative stay with him is not a concern to the association as long as they also follow the rules of the association. Typically issues revolve around parking, but sometimes when homeowners rent rooms, it becomes messy. If they do not have a written rental agreement, then I would urge that the homeowner and the association to look upon the situation as a "roommate" situation and that anything that happens is the homeowner responsibility and not the Association's problem - including violations. However, if there is a written agreement, then he may have violated the association bylaws and it may be in the best interest of the group to take action.

Review your bylaws and the state of Illinois condominium Act. You may find your answer in either. Should that not help suggest adoption of a new rukle prhibiting renting of less that a full unit.

Condo owners and associations must follow city and state rules. Our property was forclosed and the bank controlled board now looks the other way as the management company rents units in our building short term to tourists with no concerns of how it affects the residents. 
 
 
 
Look on the internet for articles of what happened in New York City when apartment building were turned to illegal hotels. 
 
 
 
We have many illegal things going on in our association. Getting a lawyer has not helped. The bank has more money and more lawyers and cares nothing about what the lawyers of owners say. 
 
Be wary of getting a lawyer. Once you start you have no control of the phone calls, e-mails and other things that they will do and you will not know what has done for you until the bill comes. 
 
Although we live in a much smaller city, Duluth, Minnesota , we have found going directly to city offices and elected officials much less costly and hopefully more effective. We are hoping to get an ordinace passed here to stop illegal hotels before our city becomes like New York City. We are personally in danger of being forced from our Lake Superior condo homes if a developer buys the unsold units here and wants the whole property to become part of his chain of time share resorts.

You are rightfully worried about 50% of units being rented out. This would create a problem for anyone trying to sell or refinance. 
 
In this case I don't think this would apply. For financing reasons banks want to see at least 60% of units being owner occupied. The case you are discussing doesn't violate this. This person still lives in the unit and thus it's considered Owner Occupied.