I really don't see how he could pick Bonnie over Jen. Jen didn't really mess up in the dinner service and she got the disadvantage of being the first one up. Yeah, she didn't catch the missing crab, but the other 2 probably won't catch it if they were the first one up.
I am so surprised that Bonnie made it so far. I have two theories. One is that they want Rock to be the winner, so putting Bonnie up against Rock would ensure Rock to win the last challenge. The other is that they really wanted Bonnie from the start. Bonnie is undecisive and have the weakest personality. She can be push around or parade as the head chef when she is not. They just want someone pretty to show and not really needed a real chef.