Really, you have nothing to do with their relationship, even if you care for N romantically and S platonically. The ball is in their court to figure out what to do, but the truth is that S will never be happy being poly and N may never be happy being mono. If they realize this, they quite possibly could break up.
If that happens, you and N have the opportunity to explore a primary polyamorous relationship. If this is the case, then you should allow yourself to be happy. Maybe the relationship between N and S had run its course, and it took meeting you for them to realize it. You don't need to feel guilty for being with someone you can love openly, freely, and on equal terms.
In the meantime, it's probably best if you let each of them know that you carre about them and will be there for them whatever happens, but give them their space. They will figure out what they need to do without interference.