1. Try the computer with only card "A". Does it work? If yes, it's not broken. If not, it's dead.
2. Try the computer with only card "B". Does it work? If yes, it's not broken. If not, it's dead.
3. If one of the cards work, and the other doesn't, ask the customer if 1 card is enough for them. If not, ask them about the overpriced one on eBay.
4. If both are dead, they should be replaced with
a) One recent, very powerful card
b) 2 recent, whatever cards.
Just forget about replacing both to the same type. It's not worth it. Maybe it's not worth even if one of them is still working. It could be sold.
CPU: AMD Phenom II X4 810 @ 3250MHz | RAM: Kingmax 2x2GB DDR2 800 @ 833MHz| MoBo: MSI K9A2 CF v1.0 (BIOS: 1.D)| GPU: Asus HD 6850 1024MB (DirectCu) @ 850/1100MHz | Display: Chili Green Vision L24FHD | PSU: PC Power & Cooling Silencer 750 Quad | OS: MS Windows 95 x64 Edition
CPU: Pentium 4 Northwood S478 @ 3200MHz | RAM: 1,5GB DDR 400| MoBo: Gigabyte GA-8S661FXMP-RZ | GPU: Abit Geforce Ti 4200 | Display: Dimarson 19" CRT | PSU: Noname 400W | OS: MS Windows XP Pro, Fedora 18