I throw them away when they get holes in (sometimes not even then - they are so comfy!), but there is no logic to her throwing them away every 6 months. If they suddenly became unwearable after 6 months of washing, why not after 5? Then 4, 3, 2, 1 oh you can't even wear them for one day now.
Honestly, why are people so scared of germs. Yes there are some scary ones out there, but damn, we have survived for thousands of years without dying out and without replacing our underwear every 6 months. Hell the Victorians only bathed once every 6 months.