Landlord (husband and wife) who were renting a room for a roommate (his 1st time out of home) on a written month-to-month agreement for some extra cash in Maryland. The roommate rental agreement used is a standard template from a legal website for Maryland and reviewed it to make sure it matched Maryland laws.

After a few months, him and my wife didn't get along after several months (disrespectful to her, her friends, and at one time entered our private space - our bedroom, when drunk). He his also disrespectful to me on several occasions.

Four days ago he verbally advises us he is moving out end of month due to him and my wife not getting along and already found a new place. Advise him as per the written agreement, it requires at least 30 days written notice from the last day of the rental month, with his deposit returned 30 days thereafter (Maryland allows 45 days).

He then moves out his stuff in 2 days, and on the last day of the month returns with placing in his room his key and a lawyer's letter claiming immediate termination due breach of quiet enjoyment and illegal rental provisions, notifying us via text message with a picture of it in his room. His mom, a former lawyer, had already reviewed the agreement and found no issues prior to signing. Letter advises contacting his lawyer for communications.

Is he still on the hook for the last month? Or do I have to pursue other recourse?

If the deposit is the same amount as a month's rent, send a certified letter stating that it is being kept to cover the lack of 30 days notice, and that you still don't have such notice in writing.
Is there anything illegal about the rental situation, as the mother claims? Generally there aren't any special requirements for renting a room in a house, with shared common spaces.
As for quiet enjoyment, that has to do with the assurance that the landlord has the right to give you a valid lease that can't be infringed on by a third party, and nothing to do with 'quiet' or 'enjoyment.' Makes me wonder if the mother was ever a lawyer.

I think joy may have misread. The way I read it, the mother approved the lease, but another lawyer wrote that letter.

In any case. I agree with her advice. Send him a letter stating that the lease he signed required 30 days written notice. Therefore, he is responsible for the December rental. Accordingly you are applying his deposit towards that rental and therefore it will not be returned. Send that letter to him at your address (I'm assuming he left no forwarding address. Send a copy to the attorney.

If he sues you for the deposit (which I doubt) I think you will win.

Deposit was 1 months rent.

Roommate rental agreement outlined notice, late payment, what was his space, what was common area, and services and utilities included as rent.

Required at least 30 day notice in writing by either party before end of the rental month.

Like I said, you will win.