Pardon the incredulity but it sounds like you don't need to replace her shower door for her. Things break, you were doing a good deed when it happened so it is not your fault.

If you are feeling guilty about granny not getting to bathe, buy her a shower rod and curtain and wish her the best of luck. I mean you can replace the broken door out of an abundance of kindness, but you really shouldn't feel obligated to do it.