Help with credit problem

[Rayjunior Nunez]

my 740 credit score just went down to 680 because I was previously with T-Mobile with a friend, in my name and I left T-Mobile after paying everything in my name, my friend left too but he didn’t pay the phone now I have $600 in collections and the one derogatory Mark. what’s the best way to handle the situation. Thank you. I did dispute it with credit karma and my friend said he’s doing things on his end to Take responsibility. Also, how will this affect my credit score if everything goes good ?