TURN YOUR BACK ON means to ignore or forget someone or something, especially a friend who needs your help.

For Example:

He needs to learn that you shouldn’t turn your back on those kind of responsibilities. (Meaning you shouldn’t ignore your responsibilities).

Real friends and family love you and they rely on you, so when they need you can’t just turn your back on them.

“You know Gavin’s OK. I remember when he was in trouble, I lent him a lot of money with no pressure to pay it back quickly. 6 years later he was still grateful, and didn’t turn his back when I needed help.