Do you think he has stayed on his meds and finally is stabilized? If that's true,it's alright to stay on decent terms,you don't have to be fond of him,but if he was truly derailed before,not on meds,abusive,etc. and now isn't,this can be the "real" him,a nicer person,etc.
I'm not saying you need to be real close and all (help prevent yourself from repeats)but it's not a bad idea when around him,to appear you let things pass.
In the past when my brother had some setbacks,we forgave him,it is difficult and has to be from the individuals he or she effected it,or one doesn't have to at all,depends on what kind of relationship. I know my brother isn't himself when in a manic episode,I know him when he is stable,like the brother I know for 49yrs. Fortunately,he has been stabilized for about 30+ yrs and things have been good.
I hope this person,will stay with a program,cause treatment is possible with great outcome.
If he is being nice to you and others,I'd say do whats simple,be pleasant in return. Pointless bringing up past stuff to him,wouldn't be productive or allow things to move forward.