2002 GMC Sonoma

A couple weeks ago my truck would not start. My neighbor came over and said he had a way to test it to see if it was the battery or alternater. So I jump started my truck and he took off one of the battery cables but my car kept running, he said it's not the alternater. I got the battery tested and it was dead so I got a new one, but the mechanic I took it to said my neighbor could have wiped out my computer by doing that. Which brings me to my current problem. I was driving it and all of a sudden my inside lights(brake, service soon, oil.) And odometer light started flashing on and off, and gauges went down. I went home turned the car off and it would not start again. I tightened the battery cable and the lights came back on so I started it and on the 3rd try it started. What kind of problem does this sound like?Did my neighbor do any damage? It only has 39,000 mileage. Thanks