Well I'm pretty sure since he's donating the computer they'd like a HD to be present, otherwise he could whip up a batch of thermite
That said, did your mom do any "sensitive" stuff on the computer? banking, buying stuff with credit card, etc? if not, format the disk and be done with it. I know my mom downloads recipes and knitting patterns, and my dad plays solitare and that's the extent of their computer computer use.