the airlines will often ask for proof of travel, because if they don't and you are refused entry, the airline can be liable for the cost. one time China air required me to sign a waiver, out of probably 12 one way trips... easy solution is to use the onward ticket deal, just before your departure. 10-15 bucks gets you a legit reservation, then it is canceled within 2-3 days.
I have gotten one by mail from AAA in the usa while living in Thailand. there's a branch in florida that offers this service, you send a copy of your license with their form, and 20 bucks, they mail to you in Thailand.
did the person leave the country while visa was still in effect? if so, I don't think there is any violation or potential fine. but maybe I am not clear on the details?