If she had found the money and asked you to buy extra food, would you of?
You need to work out if it is because she asked you to buy something which you consider wrong.
You are not the keeper of her morals, just because you help her out.
The help should not be conditional based on your opinions, it would be different if she asked you to score drugs, but anything legal that doesn't directly impact on her parenting, is her choice, because she is an adult who has capacity.
Personally i would be having words with her as to why she didn't plan for her child's Birthday, i couldn't be friends with someone who didn't do this, unless they were Mentally unwell.
You are entitled to refuse to help her in any way, if her child is suffering, then she needs outside help.
Smoking is her choice, though and being unable to leave the house, doesn't mean that she loses the right to choose.