Ok, so this is how it goes. He met someone at work. He told her that he was in a relationship but that you have now broken up. He has been claiming to work extra hours but actually he was with her, or if not with her, he was working extra to spend more time with her. He doesn't want to be dropped of right outside of his work because he doesn't want her, or one of her close colleagues to see him with you and therefore question his supposed break up.
Some colleague is outraged by his behaviour, especially the lies about having broken up with you. They don't know you well, but they do know they you are still together, maybe they asked your OH and he admitted it but asked them not to say anything.
They have been waiting to tell you because they were cheated on themselves and they don't want you to be treated like a fool. They've asked your OH some questions indirectly to find out what you live, but not obvious enough for him to guess why they were asking, hence getting the right street, but wrong house.
He annoyed that person and they admitted what they've done although he is hoping that as it's been given to your neighbour, it's gone to the bin. Still he knows he is now walking on thin line and doesn't want to lose you, so decided it was time to make a fresh break and find another job.
Unfortunately for him, you got the letter before and he stupidly left that letter which he thought would entice the lady having tried to write it in her native language, until he realised how daft it was and so kept it in his bag...