Yes mj knows that kala wants taara dead but what can he possibly do ... other than jail break
Tara made a huge mistake by not informing JR beforehand and Nandu and RP are going to capitalise on this and turn everyone against her. However I feel that MJ will do something because he knows who Kala is and he also knows that Kala has been trying to kill Tara from the very beginning. What he will do and how he will help is the question haha