Yes - I should charge you for this tip! You need to buy Refectocil blonde, follow the directions and apply this first on its own for 2-5 mins depending on how coarse the hairs are. Then remove and apply your tint. The grey will absorb a bit more tint, but it is tricky to cover grey brow hairs.
I usually tweeze out the longest grey hairs, they're often a bit on the crazy side so the brow can part company with them and not look naked.
Your client will look dreadful if you go too dark - think drag queen. Same deal with lashes, never use black on a guy - when I hear hysterical cackling from a treatment room I know it's a therapist who has ignored my advice and used black on her partner's lashes...
