I have a couple of the FTb QLs and they are a good camera considering when they were made. They're a fun, easy camera to use even though by today's standards they're pretty bare bones.
As far as the mirror is concerned, your best bet is to get it professionally replaced. It may cost you the $170 but you'll have a camera that you'll have no fear in using and that'll most likely be quite reliable for a long time to come. In your neck of the woods, try giving
KEH an e-mail as I understand they have a pretty good service dept. and they'd most likely service your camera along with replacing the mirror. Another alternative is to send your FTb to Canon. I did that when a couple shutter boxes wore out on a couple early TXs.
If the mirror pieces are still tight in the tray and not missing any glass, you could still possibly use the camera if the crack isn't in the center of the mirror. My big concern would be the mirror coming apart and getting in the shutter curtain or other mechanisms and causing a lot more damage. It would only take a sliver of glass in the wrong place to ruin the FTb. If the FTb has been sitting for a long time, it would most likely need some servicing before you use it anyways.
After I started getting serious about photography, I pulled my late Dad's cameras out of retirement and had them overhauled and/or serviced. I get a special feeling when I use his 620 film camera gear that makes the cost of the work worth every last cent.
Best of luck with your Dad's Canon.
Ernie