as long as the bb is cdma...which are verizon or sprint... metropcs will activate it. before you buy it...i would tell your friend to call the provider it was on and have them give you the unlock code or have the store do it. a recent lawsuit states that verizon and sprint must unlock a phone if its requested.
You will probably get calling and texting service. However, I don't believe you will get the e-mail or BlackBerry browsing, as those things require a BlackBerry data plan, which MetroPCS doesn't have
This link will show you how to flash it.