e client, in a non-threatening way (if you both have not gotten to a hostile place yet with this issue) that you will be informing this candidate of the current situation and that could cost them a hire. They know they are pulling one over on you since they never knew about this candidate's resume being submitted beforehand by an internal employee. They have spent time, as well as you have, in qualifying this candidate and moving on an offer. They are not going to want to lose that. If they care about hiring this person, they will have a reaction to you having to inform your client(candidate) of this situation. It is the professional thing to do as you are working on their behalf to secure them a position with a good company. The client will at least realize then that you will not go away quietly by letting them steal your candidate.
Best of luck, do let us know the outcome.