So I witnessed a player opening a sealed Khans of Tarkir booster box from a sealed case and pulled out about 8 rares from without the foil stamp at the bottom, the rest of her rares all had the stamp. The player asked me if these cards were tournament legal and I advised her they were most likely not legal as those stamps, to my knowledge, are supposed to be an anti-counterfeiting indicator. I've advised her to contact wizards but my questions is: Is this a known issue with Khans of Tarkir cards and are these cards in fact not legal for sanctioned play due to the missing stamp?

If they're real cards, then they're tournament-legal, foil stamp or no; stamps are not required in order for the cards to be legal.

The lack of a foil stamp is just a misprint like any other, though one that will probably attract a bit more suspicion than most and maybe a counterfeit investigation or two, so the player might want to not use them in tournaments anyway to avoid potential problems.