Who's Ball Is It??

[Ron_B]

High School - Sub Varsity
I was asked about this one today and am wondering for sure how to rule on it.
A is at thier own 20, 2nd and 5. On a sweep around end about 10 yards downfield B facemasks (5 or 15)and as the play continues downfield A gets hit and fumbles at the 50, and B recovers.
What is the ruling here? Remember A losses the ball after the face mask by B.

Thnaks for your input.