hcg1553 - I'm so sorry you're going through this, but this is one of the best places to find as you are. There are many caring people who have been through this here and who will give you advice and support. This site itself is an excellent resource and there are recommendations for books, etc. So you WILL get help and caring here for this nightmare that's been thrust upon you.
As for the specific problem right now, you probably already know this, but let me confirm it. This woman does not mean you well. She's not trying to be kind or helpful or informative. She doesn't feel sorry, blah blah. Her info may be very useful at this point and that's what I'd take it for, especially if he's been lying and gaslighting as he has. You can't rely on him for the truth and it's good that you have people around you who WILL tell you the truth about his actions.
I haven't been through this with an AP, but the motives are not good. It could be many things - she could be pissed off that he's not with her, or maybe he IS still seeing her and she's pissed that he's still with you, to whatever extent, maybe there's someone else too besides her, or before her. Don't be too sure she's the only one because his behavior sounds extreme from the get go - not only deserting you and your kids, but doing it at a time when your dad is very sick is extremely bad behavior. You say it's over and I think that's the right decision because I don't know how people come back from this shit and I don't think they should. There is massive disrespect and disregard here. Putting your foot down and your needs and life first IS the best way to move forward. Infidelity like this is a massive sign of disrespect and to me, other personality and behavioral issues as well - I don't think abandoning your family, even for months, is something people do in isolation of other traits and behaviors. It's a really, really shitty thing to do and it says something very basic about him - NOT YOU.
As for her, I think she served her purpose for you, in letting you know how bad things were and how much they were involved, what they were doing, kind of an inside look into how he REALLY views you and your marriage - because it sounds like he has shut down with you. You're right, she is doing this out of malice, she's trying to hurt you, and maybe him too, but I think you have all the info you need now, I don't think there's anything more useful she can tell you, and I would cut off all contact with her. It's not gonna help you or give you anything else you might need, she's doing it to hurt you and maybe him, I would just cut her off now and not talk to her or engage with her again. It's not going to get you anything else. He might take up with her again, but...you'd be in the same place you are now so why bother. You know what you need to know and it showed you a lot about HIM. She may not be telling the truth about everything....but it's enough.
At this point, I'd move forward with a divorce, you could try recon, but that goes on for years and I don't think most people heal from this kind of treatment and why bother. You've seen what he really thinks of you through his behavior, that's not gonna change. Actions matter, not talk. Talk to a lawyer and find out what divorce looks like for you and be guided by him or her. You might interview a few if it's a free or cheap consultation and find who you're most comfortable with. You could also look into counseling for yourself if that is practical and affordable, but do share what you're going through with at least 1 person you trust. Your husband will continue to gaslight you. You will have to work out living situations because...I'm sure you don't want to continue living with this jerk, and custody, etc. It's a lot, it can be overwhelming, but I firmly believe it is the best solution for most people because you cannot live with disrespect and lies.
So I would ignore her going forward, cut her off, just write down or record what she told you so you don't forget in case it could be useful, and just move forward. I'd also get a full STD panel because you don't know what he might be passing on and this can lead to serious illness. So do get that for yourself. And don't have sex with him or pay attention to any love bombing or bullshit he flings at you when he realizes that there will be COSTS involved here. I would not tell him anything you are doing - do what you do quietly and don't discuss with him. YOU take charge of YOUR life and present him with finished products. THIS IS YOUR LIFE - HE HAS NO RIGHT TO RUIN IT. Set down your boundaries and stick to them.
Others may disagree with me or have other insights, so do keep coming back and know we all wish you well and people here have many different experiences. Don't engage with AP again though - her usefulness to you is over. You know what you know and you know what it means. More will only hurt more because that's her intention. Good luck!