Hiya - really stuck on this one so please go easy on me...
ExDH of 3/4 years is a pain the arse. Still lives with his parents and hops between staying there and seeing the kids (at my place - he won't have them at his parents as it's a "death trap" and one of his parents is an alcoholic). Once or twice a week, he stays with his girlfriend who he's been seeing for coming on 3 years now.
ExH has his good points but can be a proper shitbag (hence why I walked away) and he has made zero moves to actually get his own place in all this time and to actually move on like a grown up and make a separate home to see his kids. His girlfriend would like to move in with him and eventually have kids of her own. She is mid-30's (as we all are) so time is not on her side. She has met the kids only once, 6 months ago but not since has he arranged another meeting for them all. She seems really nice person from what I know of her.
Because ex is constantly lurking around like a bad smell, my quality of life is very poor. I have been seeing someone myself for the better part of a year and he is yet to see the inside of my house because ex is there most weekends. I have no time to myself and he appears incapable of parenting our 3 DC without my help (or his mum's)! It's wearing very thin now and every time I've tried to gently broach the subject with him about getting his own place or moving in with his g/f, he loses his shit. Always has an excuse and money is usually the top one but he and his g/f earn over £100k between them so I'm not sure why they haven't been able to save up for a deposit between them? He certainly seems to have enough money for fags, booze, drugs, fancy dinners and designer clothes however! (Maintenance is often running late though! Hmmm). 
Recently, it has come to light that he has been using drugs a hell of a lot more and also cheating on his girlfriend via one night stands that he picks up - 9 times according to a couple of our mutual friends, and this was later confirmed by exH during an argument we were having about his drug use. Now as if that wasn't horrible enough, it has also come out that he was boasting recently of "landing on his feet" with this girl as she earns even more than he does and that she's so keen to push things along that she's offering to pay most of the rent when (if?) they do move in together and that he's laughing that he's gonna let her. No doubt lying about how much he's paying me so he can pretend he's got no money...🙄
God I'm so disgusted with him and horrified that the father of my kids would treat someone this way. I'm normally the kind of person to mind my own business but I just feel so, so sorry for this woman and feel like she should be made aware of what a nasty cunt he is so she is not sleep walking into a situation with a cocklodger like I did.
On the other hand, if he takes her up on the moving in offer then I am finally free of him and his endless bull shit and my life will be my own again. After such a long time. Selfishly, if I tell her the truth and she leaves him, I'll never be free of the bastard and he'll be so furious with me for fucking up his dastardly plans that he will make my life a living HELL forever more. Plus, he might, in desperation, try to shack up with someone new who is not as nice as his current lady and who might be horrible to my kids (
So to sum it up,
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tell the poor woman that she's likely being used for her wealth and exH is cheating on her ragged so she needs to get tested. I'll probably get painted as the evil ex wife but I don't care because even if she decides to stay with him then at least she will be in the know and can make her decisions based on the truth (I have text proof of two admissions of cheating although there is no proof of his potential goldigging). I will feel like I've done the decent thing by someone innocent and it might save her bacon too (and any potential future DC they might have from the pain my 3 are going through now) 
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keep quiet and let her learn the hard way (seems so harsh) and enjoy my freedom and "outward dignity" but will always feel guilty that I let another woman get played like this. I can't stand my ex and want shot of him badly. He may be stalling but if he goes ahead with her plans it's the quickest surefire way to get him out of my hair with no comebacks. However, at the cost of a decent human being who doesn't deserve what's coming her way...
So I'm really stuck between being selfish for the first time in a long while, or being decent to this woman to the detriment of me and my kids.
Both routes carry risk.
I'm not going to do anything (if at all) until the end of June, when my oldest DC finished some important exams (don't need the upheaval during that). But I'm thinking long term about what to do next. Feel shitty about it. Any thoughts welcome please!