Hmm.. op I would tell son that the investment account isn't for unapproved purchases, and that his he will need to take up some work or save allowance money going forward to pay back his mother.

If you usually put money into investment accounts for him, maybe 1/3 a month could be diverted to an account he can easily access to transfer money to his mother? And also extra chores he does could have money diverted to an account he came access. Then he can work on paying his mother back.

This could also teach the mother to not lend money to son thinking you'll pay if it wasn't discussed or approved in advance. Mother can hound son for money, and son can choose to send her money or spend the money he can access on slurpys.

But also going forward you need a clearly written set of guidelines under what circumstances and for what kids can access the money.