She doesn't get a zhonyas every few seconds. Her zhonyas only last for .75 seconds and needs to be timed really well, and has a cooldown of 19 seconds. That's quite a long window of time for counterplay. If Fiora is able to reach your ADC, then you need to tell your ADC to position better; there is no excuse for an ADC to be killed by Fiora unless your entire frontline is killed. If Fiora one-shots squishies, chances are that she is extremely squishy, and baiting our her Riposte before CCing her will guarantee a kill.
For itemization, you have to find the perfect ratio between armor and health. I don't know what it is, but you can find it out online. Since health/armor is mitigated by her passive, you as the tank (ish?) should build two damage items rather than one to kill her faster, and thus minimize damage. If you are a squishy trying to build a defensive item, raw armor should be enough to mitigate her basic attacks and lunge.