As I recall Mythbusters was able to successfully stop a radiator leak by just dropping the white of an egg into the cooling system. If I had that heater core problem and didn’t want to fix it the proper way, replacing the heater core, I wouldn’t put the egg white into the radiator, I’d figure out a way to put it into the intake of the heater core where the hose goes through the firewall.
I don’t recall if Mythbusters tried the pepper technique but I’ve heard that can work too.
A 2000 Focus is practically a new car in my way of thinking. It should have plenty of miles left as long as it’s had the basic routine maintenance. If it is in pretty good shape otherwise, were this my problem I’d replace the heater core is all. And it may turn out I’d discover it isn’t even the heater core that’s leaking. It might be a hose connector to the heater core has split.
Also, good idea to make sure that it isn’t the windshield or door that is leaking rain water onto the floor. My truck had what appeared to be a heater core leak one time, when it was actually some drain vents had plugged up.