Hi there!
Vitamins and other items can only be given to cats by the owner.
Vitamins and other consumable items are given to cats by rezzing the item next to the cat and left clicking on the item to get the menu. It will ask what cat and once you confirm which cat the item will be used on the cat.
If you are wishing to heal a friends cat and they haven't been online, then you can purchase and set down some kibble. In the menu you can set it up to either feed 'anyone' or 'group' cats. After 7 full days of eating the cats will heal.
I hope this information helps, please feel free to contact me in world if you have any other questions
