Many of us have explained why this might not be an option for him. Do you think that it's ok for a higher earner to use this as a reason for them to not compromise? Not saying that the OP isn't like this, I was asking her to find out.
I was unable to move out until I was in mid 20's because I was a low earner and so was the rest of my family and we split all the bills between us almost like a house share but with family.
So I fully understand he may not be able to move out BUT I would have never had a go at a family member for something out of their control telling them that they had messed up my plans by being in the house and making that person feel crappier than they already did. If I had my mum would have shown me where the door was and rightly so.
The OP had been ill for a while so it doesn't take much sense to think they may be unable to travel for 3 hours on public transport in the cold. I'm still recovering from an awful virus I picked up in December and still as weak as a sleepy kitten so unless her son has all the awareness of an unwashed sock then he had a whole week to consider the possibility of his mum still being ill and needing to stay at home and make a back up plan accordingly.
Also did he even ask her if she would be well enough to travel and show some concern for the OP being ill or was he just thinking of himself?
A night out doesn't need to cost a fortune, hell I had some lovely valentines and romantic evenings with my cash strapped DP back in the day. Cheap restaurant (even a KFC or maccy D sometimes), bad red wine and a evening in an arcade/ wander round all wrapped up before home to my room in a house full of other people to enjoy the rest of the evening/ weekend.
If he hadn't blown up at the OP like he did and had asked he if he could have access to the kitchen/ living room for the evening I can imagine most people including the OP saying "not a problem" and hiding in their own bedroom.
But because he blew up on the OP like he did then maybe [because she is ill] all her compassion for him and his situation decided to pack a bag and leave for the night.