If the ref believes it to be a foul, then it's a red because he denied a clear goal scoring opportunity. Otherwise it would have been a yellow.
Is it a clear opportunity, though? Yes, he's the last man, but the attacking player doesn't have the ball under control, and the ball is travelling at best parallel to goal if not a bit backward.
For me, he may have won the ball but caught the man on the follow through, in which case you could argue a foul and maybe a yellow. But a red feels very harsh there.