Of course they can. They need to apply for a UK Visitor visa. To get one, they will need to show a reason to return to Thailand (like a job) and enough money to finance their trip. A letter of invitation from the friends will help, but is not necessary if they are genuine tourists. Thousands of Thais visit the UK for a holiday.
Thanks. My work HR has advised just getting all our extensions to 27 Oct instead of 30 Nov. Makes sense, and at least it's only a month less. My fault for not noticing his passport expiry date!