My 15 year old DS has keys to his DF's flat so he can let himself in when he visits as often DF works late. The rules of being entrusted with the keys were that he wasn't to take his scummy mates there.
Anyway DS's DF (my exDP) goes on holiday for a week and comes back to find his flat trashed (his words) - actually not that bad, but evidence that DS's mates have indeed been there - empty cigarette packets, a bit of alcohol missing, bedroom seemingly having been rummaged through, some very minor damage and furniture moved as though a group have been sitting around. Ex DP is understandably upset and angry
, and seemingly incapable of speaking to DS about this without losing his temper big time - not violent, but just useless at resolving situations. He will IMHO upset DS and drive him further towards his mates.
I'm feeling that exDP is incapable of handling this well, but that DS needs to face up to the reality of what he's allowed to happen. I'm seeing it more as an abuse of trust by DS and a sign that his mates cannot be trusted and have let DS down, rather than crime of the century (which is how exDP will see it when he calms down).
By the way, my relationship with exDP is fairly good and DS is not showing any signs that he's worried about anything or concerned about seeing his DF.
So how do I handle this? Should I just keep out of it and let exDP deal with it in his own inadequate way, or do I have a role to play? 
Any ideas/thoughts/guidance, please.