So you would be okay if you walked in and someone was taking a leak in the sink? I mean your hands don't touch the sink.So why exactly his him cleaning his feet in the sink a "problem"? You don't actually touch the inside of the sink when you wash your hands. As long as there isn't a mess all up in there I don't see why his cultural practice presents a "problem".
I wouldn't compare washing your foot in the sink to taking a piss in it.
Besides. Your hands are more far more dirtier than your feet, dumbass. People wear socks and shoes to cover their feet... but their hands? People touch all sorts of handles to open doors or turn things on, they flip lightswitches, shake hands, and put their hands all over their keys, cellphones, keyboards, remote controls, money, and all sorts of public and private contaminated surfaces on an every day basis... all of which make your hands faaaaar dirtier than your feet.