Trouble is you just can't help some people. I voted to tell em but I don't think there is a right or wrong answer. If you have the opportunity fine but that ain't easy whilst travelling, you could cause an accident trying to attract attention.
I recall being out on ride with Beenz when he spotted a pannier resting on the exhaust. He rode in front and gestured him to stop. Ok the guy stopped and adjusted his kit but I think Beenz rode on thinking 'why did I bother'!
If the chain was technically lose for type of bike and the owner has done anything he is propbably a dumb ass anyone and probably wouldn't have reacted well. If you now owt about bikes you know if your chain isn't right.
I knew my chain was overtight after having new tyres fitted and believe I'm naff on bike techno knowledge stuff.
So don't beat yourself up about that Baph