Boil and must should never be in the same sentence unless youre making a bochet lol. It sounds like youre already done with primary which all boiling will do to your mead is evaporate all of the alcohol you've worked for off.
That being said, since you are done with primary you have a couple options, if youre worried about something funky on your hibiscus you can either rinse it off a little then add it dry hop style, the alcohol in the mead will kill off anything trying to be sneaky and live on it. or you can make a tea of the hibiscus, then you can control the strength, plus if you use a hot extraction (boiling water) you can have peace of mind about sanitation if you feel you need it.