Drug addicts don't make choices to abandon babies or friends or family. They make choices to get drugs with no regard for the consequences of their actions. It's not that she purposefully left her kid, it's that she purposefully needed to get a fix.
She might not want help. She might not see that she needs help. She sees that she is happy when high and that's it. That's the selfish nature of drug addiction.
I'm not saying to cut her some slack, but maybe have some sympathy. Drug addicts don't make their own decisions. The drugs and the thought-consuming addiction to drugs makes their decisions. Everyone deserves empathy and sympathy. Especially those that have had their chemical balances destroyed and viciously inhabited by drugs.
Instead of focusing on her selfish nature and your indignation about her actions, maybe focus your energy on how you can help her baby. Can you send them formula? Food? Baby supplies? Volunteer to babysit so her caregivers have a break? Offer a word of encouragement? Probably the best thing you can do is to stop putting down this woman in front of the people who have taken on the responsibility of raising her baby. They already know your feelings and don't need a reminder of why they are in this situation.