You do not have full, clear facts so should not get involved, imo.
I do worry about this. DC1 is 11 and hates going anywhere "boring" and not relevant to him (food shopping, GP, dropping DC2 to parties etc...) and often asks to stay at home.
I umm and ahh whilst thinking of a way to get him to come along and generally I manage to get him to accompany us, but occasionally I have left him, usually for 30 mins max.
All the while I am out I am fretting and worrying, even though I have given him the strict instructions and I am always contactable.
I always take DC2, age 8, even if its a 5 min trip to local newsagent as I just dont think its right to leave them alone, together or otherwise.
Im not worried that they will fight or anything, I am just concerned that its a lot of responsibility to leave on an 11 year olds shoulders. Plus, I have very nosey, interfering neighbours and in my more irrational moments I wonder if THEY will report me!
Odd really, seeing as I had a front door key at the age of 8 as my parents were rarely home.