This. Your DP should go round and talk to her.
Why hasn't he seen her in 10 months?
Clearly something happened. He should have cared enough to go around and ask her, months ago. Or is he not allowed to?
This sends up a red flag for me. Its impossible to say from your post, and I mean no disrespect, but its entirely possible that YOU are very contolling and you are not allowing your DP to spend time with his mother alone. Everyone's assuming that she is mentally ill, its entirely possible that you have the problem.
Very strange to me that you've let this go on so long, through xmas and birthday. Is that because you insist on being there as well? There is nothing wrong with her wanting to spend time with her son, alone. And vice versa. So, honestly, do you insist on always being included when they get together? You say you keep trying to make arrangements for ALL of you to visit, why can't he just go visit on his own? Would that upset you? Seems weird that he hasn't gone and done this already, does he need your permission or something like that? Again, don't know, but its possible the problem is you. And it may be as simple as that, and maybe if you back off, let them have some time together, all will be well.
I bring up this possiblity because I have a friend like this. She won't let her partner see his family unless she comes along. She can't see that she's being unreasonable. I'm not saying you're like this, but I'm putting the question out there.
Do you have a problem with your DP seeing his mother without you?