“It doesn’t matter what has happened for the employment to end, the relationship between boss and staff is over.”

I completely disagree with this sentiment, and I say this as a boss and an employee. There are bosses I would work for again, there are employees I have hired again, there are people of both I am friends with and can rely on for references. If my staff member had a goal they wanted to achieve and leaving got them closer to that (a new industry, a new role, whatever) then I am happy for them and all the best bosses are.

If an employee leaving you means you can’t ever have a relationship with them that’s just petty.