Praise be to Allaah.
You can buy things they need, such as food, clothing and so on, or you can let them buy things that are permissible, and pay the bill for them, whilst continuing to advise them, warn them, and call them to Islam, because your neighbours rights over you are great indeed. The Prophet (peace and blessings of Allaah be upon him) said: "Jibreel kept advising me of the rights of neighbours so much that I thought he would make them my heirs." (Agreed upon).
And Allaah is the Source of help.