You can't rule out that the problem is a motherboard problem if you don't test with a known good one.
It is kinda unlikely to be a motherboard problem, but that beep code does usually mean a problem that lies between RAM and Motherboard and if you already tested the RAM...
If you don't want to switch out motherboards for a known good, you could also try some other things
1) Disconnect everything except the PSU connected to the motherboard twice and the processor and 1 ram stick connected into the motherboard, nothing else. See if this result is the same, then try the other RAM stick in the same slot. Preferably your RAM instead of his.
If you can manage to get something other than 2 beeps out of the computer doing that, then say so.