Foul on player who is OB

[sj]

This almost happened and got me to thinking. I had a BIB and the touchdown had to be brought back because B1 was still in bounds but what would the ruling be in the following?


2nd and 10 for Team A at the B40 yard line. A1 is running a sweep. B1 is in pursuit from behind. They are both making the turn upfield at the sideline and A1 is now heading straight upfield. However B1 rounds out his turn in such a way that he goes out of bounds at the B35 yard line. B1, while still out of bounds and still definitively acting like he will try to make the tackle, is blocked in the back by A2 at the B32 yard line and A1 continues on for a touchdown.