Less or Fewer?

Q. Hi. My fellow editor and I disagree about the use of “less” and “fewer” when talking about percentages. In the example he asked me about, the first phrase of the sentence said something about a certain number of women who do something, then the following phrase said something like “less than 5%” did something else. My fellow editor says that because the sentence is referring to (multiple) women, the phrase should say “fewer than 5%,” while I believe that because the sentence is talking about a percentage (a single quantity), it should say “less than 5%.” None of our sources is helpful with this question of whether a percentage is a single quantity or a number. Please help straighten us out . . .