and you do NOT have a 12-month visa. There is nothing such. You received a "1-year Extension of Stay Permit". You are in Thailand on a "stay permit", not on a "visa". Check the stamp you got, what does it say? It says "extension of stay permitted until" and a date
because when you hold a VALID visa, Immigration will NEVER issue any other category of entry. You cannot legally hold two different visa or two stay permits at the same time
this is not possible. Immigration at the airport will see on the central computer that she has been issued a tourist visa, and they will insist she gets stamped in on it. She cannot just ask for a visa-exempt entry when she already holds a valid visa