There are two clues that may explain the phrase left unsaid by George Harford. (Ultimately, the interpretation is up to the individual viewer).
Early in the play, he says "...I don't believe in the existence of Puritan women..." I think he's become so used to his position in life and his decadence that he sees all others, including (and especially) Rachel that her protestations to the contrary, as well as her refusal to let her (and his) son become his secretary that she must be just as base as he views the rest of the world.
The second is in the first draft of Wilde's scene... In that draft George reacts sharply to Rachel's slap and states "... You are the woman whom I did the honor of asking to be my wife. How foolish the wisest of us are at times. But some day your son may call you by a worse name. He has my blood in his veins as well as yours..." (My emphasis).
In the final version of the play this rather hateful set of lines is excised and Lord Illingworth simply shrugs of the slap.
I think it plausible that Wilde intended for George to finish the line with "...and his wh0re..." or some such derogatory remark. He is clearly frustrated at not getting what he wants from Rachel and he's certainly not used to that possibility due to his station in life. Others are more sympathetic an finish the phrase with "wife", but I don't believe that was within Illingworth nor Wilde, for that matter...
Interesting question... thanks!