It all depends if he has the slots to use it and if its compatable Ram. Need to know the Ram Spec. of the 512 and the two 256s and the motherboard model if known.
Well if its the same kind of Ram Like DDR or DDR2 and fits the slots, no it wont hurt it. It will either boot and work or will just boot and show a smaller amount of Ram or just wont boot at all. Just be carefull not not try to cram a stick of DDR in a DDR2 slot.
If it's old old, it's probably going to be SDRAM... but just take the new stick out and compare the pins... if they match up perfectly, you'll be able to use them...