] A smarmy way of telling her that he is not interested, and at a time when she is still probably embarassed by what happened.

If Frederick thought that Louisa was already in love with him and he kept away from her as he did, then it probably could be called "smarmy".

If Frederick did not think that Louisa was already in love with him and that he knew he certainly couldn't have loved her, then keeping away from her is not actually a bad idea.

If Frederick did not consciously pause and consider whether Louisa was in fact in love with him or not, then there is no agenda behind his visit to his older brother and could hardly be called "shameful" at all.

If Frederick was confused about his own feelings and the state of the affair at the time, and felt the need to get away to work it all out, there is nothing wrong with that either.

If you think it is reasonable to think that Louisa was embarrased by what happened, can you not make the same concession for Frederick? Surely that he failed to catch her was the cause of her state. He would have been the more embarrassed of the two.

I don't list the above possibilities as mutually exclusive, nor is the list itself exhaustive. I think Frederick's actions might have well been the result of a combination of the above explanations or some that I haven't listed. But I think the case you presented is merely one possibility out of many. There is no reason to think that it is the definitive one, and I cannot find many supporting evidence for it either.