If it were me, with my French background, I would use "is" because it's referring to the sentence subject "one in six".
In French, both would be okay, even though one way may be more right
For example, we can say
The majority of the readers agrees with the publisher.
grammar wise since the subject of the sentence is "the majority")
as well as
The majority of the readers agree with the publisher.
since the theme of the sentence clearly states there are more than one reader).
Now French and English grammar rules can sometimes be very close, however I won't say that what applies to one stays true for the other