how do you disconnect a gmail from a reset galaxy grand prime without access to the gmail?

[AC Question]

i was helping my friends mom because she wants to sell her galaxy grand prime. Im not very techy but i knew she should factory reset it. I decided to go through the startup and make sure it deleted everything when it asked me for a gmail that she dosent remember the info to. i asked her for the username so i could do the forgot password, but she dosent remember it. i tryed the accounts attached to her other devices but non of them work. i guess this is kinda my fault, but how do i fix it.