I would think that would depend on management style. I would go for gentle at first.
I think the OP is talking about one employee, hence the apostrophe. In which case it should be "...to an employee who comes in late".
Apologies. I read this on my tiny phone screen so didn't spot where the apostrophe was. I agree that it appears that just one employee is at fault here. Still, we'll find out when the OP attempts to write the message. ;-)