I'm back from work. Read all your messages, thank you for all the handholding. I have such a headache. Work was OK, took my mind off it all a little, although my boss (who is also a good friend) sensed something was up and said quietly, "is everything OK?" just before my last class and I burst into tears and mumbled something about a big row with my mum. She went and started the class for me while I washed my face.
To update:
She stormed off to the pharmacy and then came back. I went in to ask if she wanted a cup of tea as I've been trying to be civil and polite but firm about not backing down. Said she wanted to talk to me and basically said that she wasn't the type to kill herself but this was driving her to it. I said there was no need for that but she had to stop saying DH was a bad dad. Denied she'd said that. I said "well, you don't think he's a good dad", and she admitted that no, she didn't but she'd never said shit dad. What's the difference? Said he uses the laptop too much, I said he uses it if DD is napping and you're playing with DS and asks at frequent intervals if all is OK, you say you're quite happy. He uses it sometimes when they're watching tv and you're wittering talking away at them. I said, it's all irrelevant anyway...he's their dad, it's his decision to make. I told her she had to butt out and stop coming btw us. Are you calling me interfering???? Yes. That seemed to send her over the top. Said, "I thought you liked me helping, I thought you liked talking to me about it" I said that doesn't mean you can criticise DH and tell him what to do. Said at one point she does all the childcare???? WTAF?
Said she'd have to leave as I'd shamed her in front of DH, couldn't live with interfering branded across her forehead.
Started storming off, saying she was going to DB's flat on the coast. Said "you were happy enough for me to interfere when you wanted help with a new baby". I got annoyed then and said, "that's it, throw it in my face" cos she does that, helps you then throws it back at you later. Then she said something really martyr like (I can't even remember what) and I said "that's it, play the martyr". She shouted bitch at me and "you're nasty". all this in front of the workman who'd come in to fix the leaky pipe in the kitchen. Not my finest hour.
Said she was going and wanted the car keys. I thought she meant there and then so said I'd take the other car to work and she said, "oh, you want me gone now?", right, I'm leaving before DS is back from school. I went to fill the car up for her and she drove off. I said 3 or 4 times (calmly) that she didn't have to do this, that she just had to get off DH's back...No no, you've said I'm interfering and poison (never said that) I have to go.
DS came home about 30 mins later and wanted to know where she had gone, so I told him for a holiday. He cried and then kept asking, will she be back at teatime, at bedtime? 
DH said DS was ok at bedtime but that he's been very huggy and saying he loves him all afternoon, so I know he senses something's really wrong. Like, he thinks someone else might disappear .
Spoke to DB who was really angry with her, but we've agreed he'll not get involved atm. Sent her a text at 3.30 which said I'd rung and not got thru and hoped she'd got there ok and that I loved her. Got one back at 830 saying she'd arrived but couldn't light boiler so not hot water. Sent text msg instructions, she responded, couldn't read msg, doesn't matter. I sent the msg again at 1030 and she sent back doesn't work. doesn't matter. Guilt tripping again.
I would like to point out that she left here with 2 ikea bags full of stuff, and a 6 pack of uht milk, drove 90 mins and apparently carried all that from the underground car park to the flat.
I cannot believe it has come to this.