Hi honey
You are obviously worried about this from the perspective of your partnership - right now I would aim to get him to think of this in practical not emotional terms.
He needs that time for marking and planning to ensure he completes his nqt year. He also needs some help obviously with his planning so that his lesson observations improve.
Can he ask his mentor or subject colleagues to meet with him at lunch to help him? If he goes to see a range of people who have good practice as well as his mentor for ten minutes at the start of lunch he will be out of his classroom and then can go back to complete the plan so he has a task that fills his time.
After school why doesn't he start to bring his marking home so he can relax and make a quiet working environment for himself (I mark at home on the kitchen table I really prefer it as otherwise I'm always interrupted)
He needs to pass nqt if he still can and then perhaps consider if the school he is in is the right environment for him or if he needs to rethink
Notice I haven't mentioned this girl - it's easy to put the inappropriate relationships label on things now I heard of a teacher who was warned as she mentioned her son who several of her students knew but she was a single parent - you do have to be careful
It will be difficult for you to raise but perhaps you need to insist that your dp refers her to the pastoral care team if she has issues and if she doesn't as she doesn't take his subject tell him that he needs the time for other things and if he carrys on supporting her then he is letting you down as it means he is not working for you as a team
He needs the help and support of his colleagues to get through this and all his time available if he's that far down the road to failing it takes a lot to turn it around but you can do - often if you involve new colleagues teacher coaches Asts you can see nqts begin to get it and become excellent teachers
Hope that helps honey 