My first guess (without knowing which algae killer product your friend used) would be lack of sufficient aeration. See, when the algae dies it uses up all the oxygen in the water, so without supplemental oxygen they fish essentially suffocate. I use the algeafix in my pond and have for years, but I always add more aerators when I do until the water clears. I have yet to lose any fish from it. Good luck, and be sure to tell us what you find out.
Joe