how did he wire up the new radio? did he use a wiring diagram for the car or did he just look for wires that seemed to match up for power and grounds? Hopefully he purchased the stereo wire harness adapter that was made to fit the new stereo to the original factory wire harness connector. If not he may have tied in to wires that appear to be ground circuits but become live when you turn the car on.
Unplug the new stereo and see if the car starts. Make sure and check all the fuses.
I had a person that installed a radio in a Honda
Accord and tied his ground wire to what he found as a grounded circuit. Problem was that the circuit was the parking light circuit and the ground he found was because the lights were off. When he turned the lights on the power from the parking light went right to the radio ground. Blew the fuse. So he put in a bigger fuse. Burned up the headlight switch and smoked the wiring from the switch all along under the dash pad then to the parking lights. The wire melted into other wires in the loom. Took hours to separate the wires and insulate and replace all the wiring.