I'm not convinced that this was on purpose. How many times have you accidentally (seriously) banged into someone rather hard because you were unaware of them being behind you or whatever. Even if it was on purpose (in my opinion-I AM NOT AN EXPERT) I think that the friend gave up any rights to a "lawsuit" when he declined a write up. Why would AA cancel his return? Even IF they did, if this man is a seasoned traveler who is well off, wouldn't he know what to do? That seems like quite an irrational idea that writing up an incident where the man is clearly not at fault would cause him to be stranded in a "foreign county"! What could he possibly stand to gain from suing this guy? As far as I'm concerned, there were no serious injuries that required money to be spent.
The other day, a guy was taking his suitcase out of the overhead bin rather forcefully and it fell on my shoulder and bruised me...he had acted like an a** but I'm sure it wasn't on purpose and even if I thought it was I would never even consider taking legal action. But hey, if the friend has plenty of money and feels like flushing it down the toilet over something like this then who am I to judge? I don't have that kind of money but maybe if I did, I would.
Please don't take this as a lack of sympathy. I am sorry it happened to your friend and I must say that it's quite impressive that he is that old and able to travel by himself.