I dont say that Emma is right in waht she does, obviossly she isn't. But if people never did anything wrong, there wouldn't be much of a plot for a novel! Not ot mention that human nature is very prone to error...
And Emma is not denigrating him as a person. She's not saying he's dishonest or mean with money or cruel or very selfish.. she's just pointing out points of difference and exaggerating them. Its true that he's not as "genteel" as real gentlemen..
And to be honest if people never said anything for fear of its being repeated or misinterpreted or passed along and changed... well conversation would almost cease.
all Harriet has to do, if asked something would be to say "I haven't been invited ot the Martins", or "Im afriad that My engagements with Miss Woodhouse take up most of my time now..."