For $50, sure, that's like 1/3 of MSRP for that card. It's a sort of lower-mid range card, but for $50 you can't go THAT wrong. You could probably turn around and sell it for $75-$100 on eBay or Craigslist without much difficulty.
Also, do your friend a favor and tell him not to bother with the whole bitcoin mining thing. Unless you have dedicated hardware, by this point, you'll be lucky if you mine enough to be able to pay for the increased electricity bill. The part most news stories about bitcoin leave out is that each successive bitcoin is harder and harder to complete. All the easy ones and then some have been mined already. Plus the Mr. Gox story from today should be enough to give everyone a moment of pause. Whether or not you want to try your luck with some other bitcoin exchange is your business, but it kind of exposes one of the fundamental flaws with digital currencies. You can't stuff it in a mattress, it's not FDIC insured... You might say that with credit and debit cards we're basically using digital dollars (or pounds or euros or whatever), but if you lose your debit card the money in your bank account doesn't go poof along with it. An ill-timed HDD crash could result in you losing a bunch of money with bitcoin, like the story of some guy who tossed out a HDD with a few hundred grand worth of bitcoins on it.
If you want to plunk down about $20K on some dedicated mining hardware, you might be able to recoup your expenses and make a small profit. Trying to just use a single video card and CPU on a desktop computer and you'll dig yourself a very deep hole when you factor in the cost of electricity. If you're going to waste CPU/GPU cycles on something, waste them on Folding@Home or Seti@Home. At least then you might be part of helping to find a cure for some types of cancer or even prove that there's intelligent life somewhere other than Earth.