Bangkok immigration office should extend it no problem.

I think the form she'll need to complete is an Extension of Temporary Stay in The Kingdom Application Form (TM. 7)

This is the same form a non-O visa applicant uses to extend their visa for one year. Even if she is only applying as a tourist for up to 30 days extension, the fee is the same ... 1900 baht.

Immigration Bureau
507 Soi Suan Plu
Sathorn Tai Road
Bangkok
10120

Tel: (02) 287 3101 to 3110

Website: Location