what does Two-bit do to make sure that johnny mother doesn't see johnny ?

I think that you are referring to Ch8. The boys are all at the hospital while Johnny lies dying. On their way out the cross Johnny's mother. She has a small tirade about how it was their fault that Johnny is in this pridicament."No wonder he hates your guts" Two-bit retorts. Two-Bit hopes the exchange was enough to make the nurse realise that a visit from his mother would be toxic for Johnny. Indeed, Johnny does refuse his mothers visit and the nurse complies.