Hello! Welcome to pettalk! I also own a pair of zebra finches, they are lovley birds. Sorry to hear about your friend's finches. Zebras breed very quickly, so if you have a male and female I would reccommed separating them. My first 2 zebras that I got had 4 babies, 3 boys and 1 girl. What ends up happening is that the males fight over the females. I lost 4 birds because of that, and now I am only left with two. It is very sad.
Id like to hear more about your finches, do you have any pictures??