Is it difficult for a friend who is a Thai national to visit UK? 

As the British Embassy do not deal with visa applications I will move this topic to the Visas and Migration to Other Countries forum.

No, providing they can satisfy the Entry Clearance Officer, the person who makes a decison on the application, that the applicants proposed trip is genuine, affordable and that they are going to leave the UK at the conclusion of their proposed visit, then Entry Clearants, a visa, will be issued.

 

Thousands of Thai Nationals successfully apply every year, many funding their own trip and some with a friend or relative providing financial support.

 

https://visa.vfsglobal.com/tha/en/gbr