What an amazing gessture, being prepared to express breastmilk for some-one who can't provide enough for her own baby.
Your body is doing a fabulous job nourishing your baby. The evidence is before you eyes - your baby. However, your body didn't know it was required to feed the whole neighbourhood. It's just feeding your baby, tailoring the amount it makes exactly to your baby's needs. And it was working beautifully. Expressing has made you second guess the situation. Of course you don't need to take supplements to feed your own child - not even anyone else's.
Read this carefully: Milk doesn't *just dry up*
It's a myth.
When your baby removes milk from the breast you make milk.
If you want to express for this other lady, your body will eventually get the message to make more milk. You will also get cannier about when/how to express. You may find it easier to feed her one side and then express the other - so that you baby stimulates your milk ejection reflex. You might find it easier to express after your baby has had a long sleep - when your breasts are fuller.
You are doing a great job, you don't need to drink copious amounts of water and you don't need do anything extra to feed your baby.