UA's companion passes are not free. There is a fee but the employee has to pay it. Usually it's deducted from their paychecks. Please offer to reimburse them!
Call up UA's revs office when it's quiet (late at night, especially on the East Coast or on a Sunday) and explain you have a buddy pass. Once a girlfriend looked up a flight from London to Colorado and it was much, much more expensive than what it cost to just buy the ticket on her own. Needless to say, she didn't use the pass!
Also, remember it's standby. Once I was stuck in Sydney Australia for a couple of days. A few employees and/or their spouses had to fly back for work and they ended up having to pay for "real" full fare tickets to do so. If you don't get on the plane, you can end up having to buy a ticket anyway, that would eat up any savings you made on the trip over. You can find yourself at the airport with 30 other standbys trying to get to the same place, most of them priority over you (if their employees, spouses or parents, for example).
I've also had friends who didn't go at all because they couldn't get on the flight. You don't get any priority for another flight and no discounts on other airlines as the employee himself would.
I used to offer my buddy passes but tell people, take these to an agent and tell them, do as well or better as to avoid flying standby. Almost always they came away with a good deal!
I really only recommend using these if it's low season. This doesn't automatically mean summer but I would say to not use them at all during those months. UA actually "blacks out" dates sometimes because they have so many real passengers, their ground agents don't even have time to check-in and process standby travelers. You will be turned away at the airport.
Also, "low season" where you live, might not be the same for other places. For example, you might think that February is a quiet time to go to Europe but a few countries have school vacations then and you could find out the hard way that the day you decide to fly to Paris is when all the French families are trying to get back from vacation. Australia also has a very different school schedule to those in the northern hemisphere.
Please remember that buddy passes are privilege and if you get snappy with an agent, your employee friend could get in trouble for giving his passes to someone unreliable. Your conduct will be reported and your friend will find out the hard way about it, even if the person at the desk or whatever were at fault.
Look into the dress code and don't get too drunk on board, especially if you're in business class (saw that once!)
Good luck and do your homework. You might do better and be more relaxed as a "real customer" than doing this.