Can they ask the university to step in? Do they have any legal recourse if he's paying his rent? He may not be doing anything illegal -- alcohol an over-the-counter meds are all legal.
They should each individually check in with their own Personal Tutors. If they can, they may want to find out the name of the Personal Tutor of their substance-abusing flatmate, and contact that academic to give him/her a heads up.
They do not have to handle this on their own, nor should they
If one of them were a student of mine, I'd be advising them to contact the Student Welfare/Counselling service. I know my university runs a couple of substance-abuser/alcohol abuser counsellor-facilitated groups. I had a Personal Tutee who attended one of these for their 2nd & 3rd year, and did fantastically well (ended up mentoring younger students in difficulties over alcohol). But only after they admitted there was a problem.
If your DS and flatmates are finding finding it really stressful, how about them attending an Al-Anon meeting?
I am once-bitten, twice shy over this: I had to deal with very bad fall out from a very ill student who had emotionally blackmailed their flatmate never to leave their shared flat. The flatmate thought they could help their friend, and their whole year went to pot. They were required to withdraw.
When someone is so ill (and hospital visits=illness in this case I think), then the other young people around them need to know the AA mantra:
You didn't cause it
You can't control it
You can't cure it
Only the addict can do that, in whatever way he finds works.
But I know when I was a 2nd year in a share house, I would have see it as important to try to help, if not "cure" an addict flatmate. I'm tougher now ...
Your DS might get information about how to deal with his flatmate's collapsing or blacking out: call the ambulance is probably the only advice, really. And be very very aware of not being dragged into the flatmate's narcissistic drama. Addicts generally need everything to be about them.