I’ve had artexed ceilings plastered over before now by a friend. Was years ago and I have no idea whether they contained asbestos or not but I do remember him hacking off bits in places with his trowel before plastering. When I asked him why he was doing that he said if he didn’t, I’d need half an inch of plaster all over if I wanted it level! Perhaps that’s why your bloke did what he did?
tidy up, vacuum up (wear a dust mask when doing so) , throw the rubbish away and get on with things best you can.
Would you care to share how it was sorted as it may help others who come on here looking for the same answers.[edit: situation now sorted]