Can I use boxed henna hair color on my keratin straightened hair?

Question

Hi, I had a Keratin straight done on my hair about 11weeks ago with the product Hair Go Straight at a salon. The stylist told me that I should not use any hair color that contains formaldehyde, PPD or matalic salts. I know that boxed henna contains metalic salts, but I am wondering if after 11weeks it is now safe to use boxed henna. The Keratin straight did make my hair relatively straight in most parts of my hair but in some parts its still very curly and frizzy and its a mess if I dont use a flat iron. The whole reason why I decided to do the Keratin straight is to eliminate using a flat iron so I can gro out my hair for my wedding and minimize the breakage and damage. Even though my hair does straighten easier and stays straight for longer in humidity its not growing longer since my ends are split and keeps breaking off and my hair is dry. Would you recommend using the boxed henna 11weeks after the Keratin straight?