I'm guessing you have 2 slots and only 1 is being used?
The new RAM stick does not have to be the same brand, but it must be the same type, with very similar specifications.
Your friend's RAM may or may not work, depending on what it is. If it is not the same type, it won't even fit in the slot.
If it fits in the slot, it still may not work. All you can do is try.
Ideally, you would buy 2 new sticks of exactly the same type and specifications. But I would try another single stick if I could, to save money.