How old is he?
TBH there are several things here:
Firstly, if he's worried about his mum being lonely/sad I would tell him that she's probably doing all the things that she won't need to do then wen he's there i.e. Going to work/tidying his bedroom etc, and I would time phone calls to her at a time when he A, is unlikely to get too upset afterwards e.g. In the morning so that there is something planned for afterwards, and B, when she doesn't have the time to pile on the guilt e.g. Just before she's due to go of to work for instance. Also, if he calls in the morning then he won't be going to bed dwelling on his mum's sadness etc iyswim.
Also I would make it so that he calls her and not the other way around. That way she can't manipulate the situation to her advantage.
Re the boyfriends, no it definitely isn't ideal if she has several boyfriends she's introduced over a short period of time, but in the scheme of things it's not the biggest of deals. They may come and go and he'll get used to it on some level even if when he's older he rolls his eyes at yet another bf being on the scene, and he will....
And don't engage on issues at home about mummy having said x or y other than to smile and nod. Don't give it the attention she wants it to have and she'll eventually learn that there's nothing to be gained by trying to poison him in the hopes that he'll bring the poison home to his dad.
It's hard I know, but you need to rise above and be the bigger people....
And this will pass. Mine are teenage now and have other issues, but every year brings different changes and what seemed like a nightmare last year becomes less important as they grow up and gain their own independence. Hth