That means 'it is better for you that he has gone'.

It's an advantage for you.

@Mina thank you Mina

in his sentence, to "be better off" means to be in a better position than before, in a better situation, in better circumstances... Here "you're better off" means, your situation will be better because he is gone (for example, you will be happier without him) I hope this helps you understand the meaning in this context.