My question involves landlord-tenant law in the State of: Wisconsin.

My question is: Can I hold a subtenant responsible for additional rent after they have moved out, since they did not give written notice.

A little background on the nature of the sublease: I am not the property owner/landlord. I have a month-to-month rental agreement with the landlord, and I am the only one who signed it. The landlord agreed to allow me to sublet to roommates without them having to sign the lease. The roommate in question, henceforth referred to as subtenant, is listed on the lease as an authorized guest (allowed to stay overnight greater than 8 days) but did not sign. I did not draft any type of paperwork to make subtenant sign, we agreed to split all expenses evenly and there is a consistent payment history of about a year of that going well. Subtenant was provided a copy of the lease, agreed to all the terms, and there is a 60 day notice requirement in the lease, for either myself or the landlord to notify the other of non-renewal. Me and the subtenant reside at the property (a house).

Now, the subtenant moved out completely one weekend (all property gone, keys returned), and wanted to write a final check covering unpaid bills and then have his rent partially refunded, based on moving out before the end of April. I didn't agree, and he then wouldn't write any check, stating I would just write him off once he paid the bills, and that I have his number and can call when I'm more reasonable. He never gave any form of written notice, and my understanding is that only written notice is valid. He did give a very flimsy verbal notice, saying he might move back with his parents at the end of summer, then added maybe he would move back there at the beginning of summer, and furthermore added if he finds a job sooner than expected he will move out at that time, if he needs to move to accommodate the job. So he got a job, and just took off. So he is paid up on rent through April, but there are some outstanding utilities. He seems like he intends to pay what he owes, but we're stuck on the rent refund (and whether he owes more for not giving notice).

I got blindsided and don't have any type of warning to try and find a replacement. Our friendship did start to take a bad turn near the end, and I'm not sure if that was a factor, but my seeking additional rent is not vindictive. If it's money I'm legally owed per the lease and WI laws, you bet I'm going to try and collect. Can I tell him that he never gave me written notice, and once he gets me written notice, he will no longer be responsible for rent in 60 days? I would not try and seek any other expenses. There was no security deposit. And if I am owed a notice and additional rent, do I have the same "duty to mitigate damages" that a landlord has to try and find a new person ASAP to limit his rent due? What if I decide I don't want another subtenant? And finally, how can I prepare for the (knowing him) inevitable defense claiming that he did give proper notice, and that I either misplaced it or am intentionally disregarding it to get money out of him?

I've asked around and done some reading, but I still need some direction or informed opinions. I can't seem to find anything that states one way or another that a subtenant has to give the tenant written notice, but it sounds like accepting rent created an implied lease with the same protections and provisions as the actual lease. I also was told by some that because I accepted the keys back and am talking final bills, it would be hard to convince a court (if it went that far) that I am expecting 2 more months of rent from him. Obviously, I learned a lot from this ordeal, the big two things being to get everything in writing, and also to not mix friendships and finances.

Any information anyone can give me is greatly appreciated. Thanks!