If you want to help the ex and you truly believe that she is in financial trouble, then why not do something like buy her groceries? I personally wouldn't give her cash directly in any form, since contrary to the other poster, I believe that could come back to bite you in the b---.
How do you know for sure that she is in trouble? I always hear from my son that "Mom has no money". Actually it only turns out that Mom has no money when it comes to our son. She has money for a new car every 2 years, a new boat, a camper and a big screen TV all bought recently ( I'm sure not actually paid for, just all on monthly payments since that has always been her thing). But son's $50/year high school book fee couldn't be paid, so he didn't get his diploma at graduation!
Also, is Mom in financial trouble due to poor choices that Mom has made or is she in trouble due to no fault of her own? In my opinion, if it is due to her choices, then you throwing money at her will make absolutely no difference. It would just enable her to continue to make poor choices. What is the debt that she is paying? What kind of car is she making payments on?
I am rather tired of hearing about the poor Moms when nobody gives a damn about the poor Dads!