oh dear. all too familiar... please dont let him take the girls.
remember - you are not repsonsible for "sending him over the edge" - that is his problem. (if he does at least it will prove a point - that he needs continued psychiatric help...)
i moved out from joint home with dcs - let my ex come to see dcs there as could supervise but he would refuse to leave, break things, throw things etc. he got worse and worse....now some time later he has supervised access, it may progress depends on him.
do you trust the friend at all?
even if you do, you need to tell your P that he cannot visit while he is unstable - but - you cannot tell him this while he is visiting as he will likely get angry. that is his problem - but you need to make sure you are safe. eg have a third party present, meet him in a public place without the kids etc.
you need to see a solicitor about drawing up a schedule of contact, preferably supervised .... eg contact centre.
can you speak to his GP about your concerns? if not at least speak to your own GP about what is going on.
in meantime - i would say -
dont have him visit at your place unless you have a third party there who will escort him out if needs be.
dont be alone with him and girls anywhere
dont let him take girls off in car etc.
speak to your GP about this. if is same GP or same practie they might be able to share what you are saying with each other....but they cannot do anything til he does flip...
in a way you might be better off saying "you cant see the girls til you have seen your psychiatrist" and stick to that.
you can tell the girls he is ill (true)
speak to a solicitor about getting supervised access -this takes time tho.
ultimately, you almost have to wait for him to flip - just make sure you and dds are safe. you have to be prepared to close the door and lock it - then call the police on him if he starts banging and shouting. then it starts off a process.... (my ex flipped - smashed door, grabbed phone off me refused to leave, held us hostage for two hours...then i was able to call polie and they arrested him..then it paved the way for court ordering supervised access given all the other abckground.
in meantime make sure someone else always around.
you could try saying "we are scared of your behaviour. so for the time being you can only visit in a public place with someone esle around. you need to go back to the doctor/psychiatrist. "
if no one around, he doesnt visit girls.
you could ring your local CAFCASS too and explain what is happening and your fears.
alos, record what happens each time in a notebook or diary eg "came round and paced up and down threttenign to take away the children..." "children were scared because..."
and you need to be calling 999 when he acts up. there must be records of his previous breakdown...