He replaced the relay yesterday and it still died today at a stop light but it generally starts right back up.
Could it be the gas you buy? We have a Murphy station that opened here a few years ago.* Cheapest gas in town. My car died twice, oddly enough heading to their pumps. Car was fairly new and so was the gas station. After only filling up twice there, I went across the street to the old reliable Shell station and only bought my gas there. Never had another problem since.
Do some research and find out which wire bundles do what (fuel injection, ignition, etc ). Start engine and let it warm up. Move bundles slightly at various places while it’s running. Stay clear of hot spots, fans, and coil wires.
Run through any high puddles lately? Are squirrels in the area?
* Was always told that it’s always the best gas at a new station. The pumps are newly calibrated and water build up has been purged and isn’t in the tanks. Not in this case.
Many times the engine management computer box will go out. I had one that did that and when it cooled down it would start again. I took the engine management box apart and found that the very high power voltage regulator that takes 12V down to 5V had very wide pins that went through large holes in the circuit board. The temperature cycling from the high current caused the solder to get brittle and crack around the voltage regulator pins (the big part, probably with a heat sink radiator and three big fat electrical pins). When it got hot the expansion would cause an open circuit at the solder cracks, but when it cooled it would shrink and make contact again. You can see the crack easily with a magnifying glass if this is the problem. I had cracks all the way around 2 of the 3 pins. I wicked the solder off with the copper braid solder wick. Then I bent the pins over so they touched te copper pads around the thru-hole in the circuit board, but not shorting to another trace. Then I re-soldered. The problem never came back. Your problem could be different, but my problem was on a Porsche and I got many calls from the repair shop to help fix the same problem on other cars including a race car that had the problem in the 12 Hours of Sebring Race. It also had the same problem. I have also heard of this in other models of cars too. A simple fix if it is the problem instead of a $1500 replacement of an engine management ECU.
Often the engine management ECU os under the passenger floor board, but you would need to check for your vehicle.