He should do two boards and see what it does. If it works and he has the correct t bitmain firmware.
He can clock -2th and try three boards.
The firmware is called "multi-option user firmware" under the name "Antminer-S9-xilinx-201907302037-autofreq-user-Update2UBI-NF-sig.tar.gz", the -2TH will put him at about 1000w, not sure if that will help, he would probably need to disconnect one board.
yeah used Apw3++ vary on what they will tolerate when they are running on 120 volts.
I have had them put out only 750 and or as high as 1050 watts.
They are good for close to 1600 on 240.
I lot depends if the 120 is doing 108 to 111 due to brown out conditions. Or if you are near 117-121 volts.
I sometimes struggle in the 850 watt range. with my L3+ when I test them in my garage on the 120 volt circuits .