I agree that grief is an individual thing. I do not believe it is terrible when a person who has experienced such a loss finds someone with whom to spend the rest of his/her life -- even if that occurs rather sooner than other people might wish -- as long as the grieving person is far enough along in the grieving process to be ready for a new relationship (otherwise, it spells disaster for both parties).

Looking at the situation in the context of JA's times:
Having said the above, I must consider the societal expectations during the regency era. Grieving periods were something almost "formal"; there were, at that time, cultural rituals associated with the grieving process. While Captain Benwick and Fanny Harville were not yet married, I think it natural that other people would have had expectations about the manner in which he was seen to be expressing his grief, given the degree of attachment that existed between his finacée and him.