Ah, no, in fact if the companion flew part of the trip and not the rest, you stand no chance and your situation becomes tricky as technically, the companion should then be charged the one way he/she flew (unlike cases where the companion does not fly any part of the trip where there is basically no recourse for the airline). I would not check the companion in to avoid attracting attention and lie low. It is very likely that nothing will happen but what you are doing is explicitly against the terms of the voucher so the less you get noticed the better, especially in this period where flights tend to be very full.