When a participant of yours leaves or is let go, I think one of the most important things you can do is make sure they roll out their assets from your retirement plan.
Sure it means that you might have fewer plan assets in your plan, but I think it’s important to roll out their assets because you don’t need the headache of dealing with former employees. You have notice requirements you still have to deal with participants that are former employees. Plus don’t forget that former employees are bigger pains to deal with than current employees. So I recommend making sure they roll out their money. If they are under your minimum threshold, then cash them out if they don’t. If they are above the minimum, do your best and try to have them move the money out.