Can a Thai apply for a new UK visitor visa before the current one expires?

That's a shame. There shouldn't be any problem in getting a new visa, but make sure you cover all the bases in the application especially her reason to return to Thailand. Some 2nd applications are rejected because they assume UKVI will refer to the first application. They don't - the 2nd application is considered on its own merits. Good luck.

If she does go back to the UK on her current visa, she will need to explain why she made a 2nd trip when she applies for her next visa. As long as she had a good reason, it won't be an issue. There is no limit to the number of Visitor visas however she can only stay for a maximum of 6 months in any one year.

If the visa is still valid she can travel to the UK again on it, as long as she leaves before it expires. UK Visitor visas are multiple entry and valid for 6 months.