What are absolute maximum ratings of FAULT -pin in AD5412?
Is it possible to connect it to +24VDC for example through 2k2 resistor + LED(opto coupler) in serie?
The /FAULT pin is a digital pin so the absolute maximum ratings of the digital output pins to GND is followed.
To answer your other question, connecting a 24V supply to it will not work and may cause damage to the part.
If you want to use an opto-coupler, you may connect the /FAULT pin to a 10kΩ pull-up resistor to DVCC. You can send this signal to the opto-coupler. Depending on your current setup, you may have to change supplies if the current is not enough.