Book your return flight 60 days later. I suggest 59 days to leave room for the unforseen. Once in Thailand, go to immigration before your 30 days visa exemption expire. You can do it even on arrival! Pay your 2K Baht and whalla... 30 days added to your stay, giving you 60 days.
You are going to come off second best, because you are a farang. Don't expect anything. Worse....she might want you to pay for damages, and compensation for her injuries. Good luck!