Progress Stopped, or Play On?

[mtn335]

Oh gurus of the gridiron, lay your wisdom on me! If the embed doesn't work right for you, the play is at 1:17:15 or so in the video.

Ball carrier A35 is hit and starts to be driven backward, and at *about* the same time teammate A1 comes in, rips the ball from him and takes off, eventually scoring a touchdown.

https://vimeo.com/51839274#t=4635

Would you have ruled this play dead (forward progress stopped)? Or would you have allowed it to continue?

You can see what my crew did (I'm the R) and on first glance I think I agree with my L's decision, but I'd be curious what the community thinks.

On a secondary note, this was just my third Varsity R assignment. If anybody picks up anything for me, please don't hesitate to pass it along. Thanks much!