He should not have been able to put RD Ram into a DDR slow as it has notches in different places.
Not wishing to accuse your pal of anything, I will say that no-one admits to doing anything wrong. Its easy to force something and to quickly wish you hadn't.
The fact that the wrong RAM was installed may not have been the reason why the mobo failed.
I expect that the only thing he could get back from company X is the money for the RAM, or the correct one. They will no doubt have a limitation cluase in any warranty/contract which means they are not responsible for the damage caused.