seller retaining earnest money if buyer backs out

There is no easy answer to your question. By the plain terms of the typical finance contingency, if the buyer cannot get a loan committment by the contingency date, they can terminate the contract of sale without penalty and get their deposit/earnest money back.
However, all contracts have an "implied covevant of good faith and fair dealing"--an obligation, imposed by law, on both parties to act in good faith vis-a-vis the other and to not willfully do anything to deprive the other of the benefit of the contract. Deliberately sabotaging their ability to get a loan or not sufficiently aggressively pursusing reasonable financing options would be a breach of this covenant by the buyer and enable the seller to retain the deposit, since a breach of the covenant is treated as breach of contrac
However, the question is, is changing jobs during the loan approval process bad faith or not? It may well be, but is not automatically bad faith; it depends on the circumstances. If the buyer left their existing job for a job with better pay or benefits, for example, that is not bad faith--it was a reasonable action almost anyone would do and would not be considered a deliberate act of sabotage of the ability to get a loan, even if it does result in an inability to get a mortgage. Changing jobs for a better one when the opportunity presents itself is not bad faith.
But changing jobs to a job that is no better could easily be bad faith, since then it may have been done in an effort to derail the loan and escape the contract; if there is no reasonable pragmatic explanation for the job change, that could support a finding of bad faith.
So the answer must be found in the specific facts of this situation--why did this buyer change jobs now? That will answer the question as to whether this was a breach of the finance contingency and/or the implied covenant of good faith and fair dealing.