So this advice is not for the probational. The way I handle that situation is with a nice long dose of sick list. If the training managers won't split you, it's time to take it to the self help level. I had this exact situation back in 2000 when I did my initial B-777 training. I gutted it out and tried to help the weak captain. He was unwilling to listen to anybody and especially not me. Long story short after 25 days of hell we were split the day before the type rating check ride. I passed with flying colors the next day and he went on to do multiple additional training sessions. Thank god he is now retired.

I will never put myself through that situation again.