My read on the play as described is a little bit different...
When no "safety base" is in use (NCAA does not use one), the runner is permitted to exit the three-foot running lane on her final stride to touch first base. (See NCAA rule 12.4.8)
When you think about it, this makes sense. The running lane is entirely in foul ground and the base is entirely in fair ground. The lane leads, essentially, "nowhere". If the runner were required to keep both feet in this lane all the way to the base, she would never be able to touch the base!
On this play, it is quite possible that the runner was within her rights to be where she was when the contact occured.
Further complicating the call...
The description provided says that the throw was "up the line" and required F3 to "reach to her left". That could mean that the off-line throw pulled the defender into the path of an otherwise legally advancing batter-runner. When that happens, the runner is not guilty of interference (unless she does something flagrant, like purposely swiping at the ball to knock it loose, or goes out of her way in an attempt to purposely collide with the fielder).
This very play is covered in the NCAA rule book, under rule 12.14.3. This reads, in part:
"Simply because there is contact between the defensive and offensive player does not mean that obstruction or interference has occurred.
Note: If both the fielder and base runner are within their legal rights, neither player shall be penalized for the incidental contact.
Sample Play: The batter-runner collides with the first baseman who is attempting to make a play on an errant throw up the line (toward home plate). The runner has a right to the base line in this case, and the first baseman has the right to field an errant thrown ball. This is the result of the "normal" flow of the game, and the play should be ruled incidental contact with no effect."
As described in the play offered, the right call here very well could be "no call" at all.
