Hang on. he was unable to walk and slurring, but in the op, she says when his mum phoned an ambulance he left the house both can't be true.
I think we're only getting a confused story, which is understandable if the Op has only heard it through her dd who was clearly upset.
As far as we can tell, the lad wasn't brought round there by the police, so I'd guess it was one of the following:
As others have said, he'd left the house, they were trying to find him and phoned her to see if he'd turned up there.
They told him they'd take him round to her, as a way of trying to calm him down. They may have had absolutely no intension of taking him there, but hoped that would calm him down enough for him to listen to them.
I wonder whether it was the police who called or him - obviously the police would have had to ask him for the number.
Or was it the mum who phoned, asking her if she brought him round could she talk him into going. Obviously this was inappropriate, but I think we can all appreciate that she probably wasn't thinking straight at that point.
So really a difficult situation in all; I don't think there was an easy and obvious way of handling it. They will not have known the situation with op's dd. It may be that she was painted a much better picture than the reality by either the chap's mum or himself.
But the ambulance do not have any authority to take into hospital or treat someone who has capacity who is refusing. I've been in that situation with elderly people a couple of times, where they're swearing that they're absolutely fine, when clearly not, and the ambulance service can do nothing.
In one case they'd broken a couple of ribs (I informally phoned daughter, who I knew, as they left and suggested she "dropped in" very shortly) and the other time they had concussion and a broken nose. Had to rush her in half an hour later when she lost consciousness.
In both cases the ambulance service was advising that they came in and they totally refused, and they said there was nothing they could do. They were clearly upset because they could see the risks, but their hands were tied.