Wording for overstaying guests

Other

Can anyone please know how I can deal with this issue in a fair way.
I had wording in my tenant's contract that for her guests to stay no longer than for 14 consecutive nights in one year or 7 nights in 6 months, as this had become an issue with my tenant and her family guests staying. My house she rents with her three teenage children is a large spacious 3 storey, 4 bedroom property. But now my tenant's very close friend, a young person has given up his flat and moved away into a small bedsit near where he works. Her friend has now become a regular guest coming to sleep almost every weekend in another part of the house. She denies that it is almost every weekend and makes the excuses that he goes to see his friends and family, which they do sometimes but then comes back and sleeps at the house. With the house being so large and the use of all the amenities etc all in full use. The maintenance and replacing of electrical items etc which I am responsible for are expensive. I am not complaining but the contract is for my tenant and her children to live there. Not to have guests living in part=time as and when and using all the facilities. So if anyone please could suggest how I would be able to add this into a contract regarding guests NOT staying more than two weekends per month. Would that appear to be reasonable. I am in the process of issuing a new contract which my tenant is agreeable to.