Hi Almondhoney,
I would wait two weeks between the relaxer and hennaing treatments. And follow the henna treatment with a good moisturing cream, butter or oil. The issue you have to worry about is not the 100% natural henna powder. The disaster you can get with your perm will come in lots of assorted commercial hair care products. You want to take care with those other products you may use.
If you choose to use a lot of different products with lots of ingredients that you can't identify, with any hair treatments, I never think it is a good time to throw anything additional in the mix...including 100% natural henna.
If you happen to pick up some other conditioner or other product containing a metallic salt of some sort in it and use it with the henna, the problem you could get is strange hair coloring.
However, if you are committed to keeping your products few and will not be adding any other sort of hair coloring you will be alright with the 100% natural henna.
HennaJoy