She’s saying it will take years to fully train someone for the position though as it can’t be just jumped into. I’d say the four months is really valuable for someone fully trained so they can at least START working with the new apprentice. It sounds like risking that two weeks is really risky.
It sounds like if it takes a year at least to train a replacement four months is highly valuable to still have a trained professional on staff. Also since employee is moving out of state, no risk of clients following her to her new position. Loyal clients might even like if she’s having a hand in training her replacement and not just suddenly gone. OP is being short-sighted especially given cash flow will cover training in one day.