By the time I noticed the trailer acting squirrely to the time the tire jumped off the axle I could not have called him by cell phone. I will also admit that I believe that they were going too fast and had unfounded faith in their trailer.
HS, yes, only 1 tire was replaced which was also a no-no. I also agree with you that the passenger tire was only 1 of several potential contributing factors to the trailer accident I described. I know that my friend had become a bit cavalier about trailering his boat and was just being cheap when it came to replacing the tire(s). This incident was a wake up call for him and he realized that we really dodged a bullet since the rogue tire did not jump into the oncoming lanes of highway traffic which could have been fatal for another driver.
I also learned a lot of trailering lessons from this incident - the first is never let your guard down or assume that everything will work as designed or intended. A standard checklist of things to check BEFORE leaving your driveway with a trailer would probably cover the rest.