@Tammy221
If it was an IB, your friend could have used one of his penalty-free cancellations by contacting Airbnb as soon as he realized the mistake. Also, I agree with @Sarah977 that since "no linens" is not the norm, even if it's stated on the listing description it would be prudent for the host to reiterate to guests.
I have a "no guest(s) of the Airbnb guest" rule mentioned several times on my listing, but I always remind guests of this rule at least 2 times (during initial inquiry and again a few days before they are scheduled to arrive).
Your friend should contact Airbnb to get the booking cancelled and have the guests re-homed. Airbnb will handle the refund.