Why does him being on the diet with you - affect YOUR journey? His successes or losses shouldn't affect your own.

You are not connected at the hip!
I know it's a bit of a downer, I do better when my OH is doing the same diet with me, but the way men's weight loss is, I know that he'll be done sooner than I am and he may start doing a more food intense variation of the VLCD we're on (i.e. 3 packs, 1 meal per day) soon, while I am going to stick to the 4 packs a day.
So, I know I can't attribute my successes of willpower to that of his own.
Why are you feeling gutted? Are you feeling let down by him not being able to stick it out? Do you feel he needs the diet and you want him to get slimmer? Are you feeling slightly jealous that he gets to be 'off it' while you feel you still need to be 'on it'?
Have a think why you feel this way.