I'm sorry to be asking this, but all the answers I can find generally deal with THC. How likely is it that a one time use of a small amount (~.25g, used about a month ago) of cocaine will show up in a hair follicle test?

The hair follicle test usually tests for habitual use not one time use. It is the same for pot as it is for Cocaine, if you only used it once then you're fine. If you use once a day for a month than it would be positive.