Khan boxed well, but I'm afraid Judah just didn't turn up. IMO once he found he couldn't do much with Khan he just folded, the low blow gave him an excuse to give in.
Judah at his best would of been a brilliant challenge for Khan, but Judah's finished now, and employing Sweat Pea as his trainer was a last ditched hope that he'd do something magical with Judah's career when (from what I could see) Judah himself already knew that he doesn't want it any more.
The punch hit the belt line which was high anyway, Judah was looking for a way out and that was that.
A bit of a hollow victory