Chances are she already knows, but is in denial or super clueless. My sons XGF told me what he was doing with the money I was giving him, and it kinda re-enforced what I already knew. I think if I ever told him what she said he would have just put it back on her, or tried to lie his way out if it.
Unfortunately, I still continued to enable him for quite some time.
Fast forward to now:
I recently let my family members know that when he asks for money it is for drugs, and so far they have been great about it, but it is always their choice.
Just be careful that you don't keep trying to save him.. you know So you can move on and heal.