Emma's with you, even though she also think Frank's at fault for not coming, she is surprised by Mr Knightley's severity:

To take a dislike to a young man, only because he appeared to be of a different disposition from himself, was unworthy the real liberality of mind which she was always used to acknowledge in him; for with all the high opinion of himself, which she had often laid to his charge, she had never before for a moment supposed it could make him unjust to the merit of another. (chpt 18)

It is suspicious that when Emma praises Frank, Mr Knightley criticises him:
"And mine," said Mr. Knightley warmly, "is, that if he turn out any thing like it, he will be the most insufferable fellow breathing! What! at three-and-twenty to be the king of his company -- the great man -- the practised politician, who is to read every body's character, and make every body's talents conduce to the display of his own superiority; to be dispensing his flatteries around, that he may make all appear like fools compared with himself!

One moment Mr Knightley is criticising Frank for not standing up to his aunt and uncle because he has reached the age of rationality, saying Frank does not show sufficient delicacy, the next Mr Knightley is criticising this hypothetical Frank for showing too much delicacy and calling him a puppy.