bff-in-law
refers to the husband or wife of your best friend forever bff. Because bffs are basically family, there should be an easy reference for their spouses. this may be gender-sensitive as more women tend to use bff than men, but can still be used to refer to a female spouse of your male bff.
I can't wait for my bff, Adina, to marry Adam. he'll be the best bff-in-law ever.