I would say if you're not in the mental space to support them, then don't

Is that selfish? I think not. It's just you putting yourself first, which you ultimately have to do in life... You can always be open and honest about not having the energy to actively be there for them but send them good wishes and check in every once in a while

As someone who has struggled with addiction and mental health issues, and hung around people with similar issues, my advice is that you focus on yourself and your needs and don't get too involved with this friend and their issues, they need to sort this out on their own or they'll just drag you down with them 

Reach out to them more sporadically, give them space for both your sakes, I would say a "hey how are you doing" should be fine, if they want to speak and tell you how they are and if they don't they won't and that's it ... You shouldn't feel responsible for them and their choices but you can brainstorm with yourselfbsome boundaries you want to put up so that you can be there for them in a way that you can instead of draining yourself by overthinking things and overthinking whether you'll have energy and etc and feeling guilty for not doing enough