Lao Lady Overstay In Nong Khai

My girlfriend can border-hop on passes using either her Thai or Lao ID cards. She can't get a Thai passport owing to current lack of a tabien baan.

Her last 30-day entry at Nong Khai expired in November 2006. She didn't return to Laos on that passport as she didn't think she would need to use it again. Change of plan. If she does try to re-use the Lao passport to enter at Nong Khai, what would happen if the officer noticed the lack of an exit stamp last November? Is it as serious as a Baht 20,000 fine, or could it be 'explained' by use of a border pass to exit Thailand at that time? Would the immigration computer (if any at Nong Khai) have flagged her as an overstayer?

If it was just a matter of not having an exit stamp, could she perhaps just apply for a clean passport in Vientiane? Or is her name, as a Laotian, dirty forever more as far as Thai immigration is concerned? Thanks in advance.

A Lao passport is useful for visiting Cambodia as no visa is required.