When Omega!Sherlock turns up on their doorstep a few weeks before John and Mary's bonding ceremony, shaking and out of his mind with heat, Omega!Mary is quite happy to help him until Alpha!John comes back from doing the shopping.

John returns home to find his fiancée cradling his best friend, sex toy put aside, both of them reeking of sex. John teaches Mary how Sherlock likes it (having helped him out for a few heats before the Fall).

I'd like to see something about John's scent helping Sherlock in itself, but it's not vital.