You need to talk with your friend. You do not fly free. Travel companions fly and the friend pays TAXES ONLY on imputed income on that flight. There is a calculator on Uniteds site to check prices (not very good). So, if your friend had $1000 of imputed income, he gets $1000 added to his check, it gets taxed and then the $1000 gets taken off his check. So he needs to know what all the taxes are on his check. for me the taxes on imputed income is up in the high 30% range so I charge my friend 40% of the imputed taxes so looking at the above $1000 it really costs only $400. Your friend needs to keep up with how this all works.
Also, United is VERY slow on charging me (I am also UAX carrier), not sure how they are at charging their own employees.