NTA, keep it secret she should love you for what you give. if you decide to invest this money and try to make something of it for yours and potentially her future too rather than flutter it away on something immediate then thats just being responsible for your own wellbeing, 135k isnt nothing but its not so much that you can just throw it around, your gran did not save hard for that money for you to just give it away.

If her dad is bad with their money, he wont be any better with yours and easy come easy go. He's more likely to gamble it to try and recover his own failed fortunes. Help them out with food and groceries if things get bad and dont ask for it back, but they took responsibility for their financial decisions when they made them. A handout wont help.