I am going to be visiting a friend who is currently residing in the Chang Mai Woman Correctional Institution and I would like to know if it is possible to bring anything to them or not. For example can I bring them snacks from 7-11? Also I understand they have prison bank accounts, I read somewhere that the maximum they can have in their account is 9000 baht, is that correct?

You can take food stuffs in, however by the time they have dissected it, there won't be much left. Better to pay some money in to their accounts, much more use to them.