A friend replaced the cv joint for me and something keeps shearing the roll pin.  It happened twice. Once I backed out of the driveway and it sheared the pin and the next time I drove around the block before it sheared the pin.  What could be causing this and is it an expensive fix?

Wrong axles..... had this happen to me multiple times, if the parts store sells you a turbo axle for a non turbo car or vice versa, the spline count will be slightly off. It will stil go on, but there will be play in it and then.... snap goes the pin.

+1 . Wrong axle.

I can also vouch for this. Autozone got the wrong axle as well bought 2 and they were both incorrect splines were to big. Ended up finding two originals on eBay.

What vehicle?  If it's an ea81 Hatch, then order parts for an '84.