My jungler decided to go full damage when we needed a tank
I didn't mind at all, he was fed early and was doing well throughout the game. Later on however when we had to group he didn't do anything except kill the enemy adc who was already pretty worthless because she was doing really bad. Later on we lost and now I'm really sad we lost because our jungler who is a good player decided to build full AP, WHAT DO I DO!!?
should build tanky because of her passive. The heal is a percentage so it will feel the same to Zac wether he was 1k HP or 4k. He gets no additional armor, or aditional HP and his scaling are not low enough to disencourage not building AP on him, if he goes AP he will do a lot of damage that is what stops people like Nautilius and Maokai to go full AP.
and start taking objective after objective avoiding any kind of conflict, if they contest an objective you instead forfeit it and trade it for something else on other side of the map.