I had a similar situation on one of my homes. I knew the boyfriend was technically living there, despite the lease limiting visitors and saying that additional occupants had to be approved. I turned a blind eye to it because she was a pretty decent tenant at that point. What I didn't know was that he had multiple children that moved in also (putting them over the occupancy limits for the town and HOA), and his car put them over the limit for the HOA allowed number of vehicles.
This ended up causing me all kinds of heartache from the HOA, and since I had turned a blind eye to it for several months, the tenant got REALLY mad when I brought it up at that point. They turned on a dime at that point and became nightmare tenants, and tore up my house.
For all the people that say "what does it matter" it really depends on the limitations the landlord has. If there are no occupancy limits, no HOA limits, or no code limitations on number of people and vehicles, then the landlord has to decide what is best for them. If any of these limitations exist, then I don't think you can just let it go, because the reality is that at some point it may come back to bite you. I have learned that the sooner you deal with the situation, the better it is. I think my tenant might have taken it better if I had addressed it up front, but since I let it go she took it as an accepted situation and then took affront to it when I decided to "change it". Even if she wanted to leave earlier in the lease, I don't think all the damage would have happened if it had been dealt with early, and all the anger over the situation hadn't built up.
The issue of the boyfriend becoming a squatter if the tenant leaves is also something I became concerned about.