Ok thanks. From what I understand, you have a Mac that is unequivocally yours — you paid for it, you own it, that is without question. But, your Mac was given to an employee who subsequently configured it as his or her own, who has since left to "pursue other opportunities"... leaving you stuck with a Mac that you basically cannot use because that person is unwilling to help. Is that correct?
If so what the Apple Store told you is their official position. They will certainly help you to erase and reconfigure and reclaim that Mac as yours, but they absolutely positively won't do that unless you can unequivocally prove to them that it really is yours. The reason they do that is to discourage theft of portable and fairly expensive devices. You will find Apple unyielding on that matter, since that was their eventual response to what was quickly becoming a vocal and shrill public attitude that turning a blind eye to theft is tantamount to encouraging it. At the time, these pages became overwhelmed with demands for Apple to "do something!!!" and their response was Activation Lock for Mac - Apple Support and its complimentary implementation for iPhones / iPads.
Word quickly spread that stolen Apple devices are utterly useless, and thefts dropped dramatically.
Apologies for a long-winded explanation you did not ask for, but the bottom line is that you will need to prove to Apple beyond the shadow of a doubt that Mac is really yours before they will be willing to help. They may or may not accept something less than a receipt or proof of purchase, but all you can do is ask.
Start here: Start an Activation Lock support request.
