I had to do this for my in-laws who wanted to immigrate.
I wrote something like:
(Headed paper with name, address and phone number on it). Date.
Dear in-law
I am writing to confirm my invitation to you to stay with me indefinitely on your arrival in the UK in Septober (or whenever it was). You and Mrs In-law are of course welcome to live in our household until you are able to set up on your own, and we will provide financial support until you are earning.
Some more platitudes
Yours Uncle Phil
In my case, he needed to show the UK immigration people that he had a "sponsor", so that he wouldn't be dependent on welfare when he arrived.
In your case, the Thai authorities probably want to see evidence that your friend genuinely is making a visit to a mate and not planning to leave for good or become a burden on the UK (or Finnish) authorities. Style isn't important, but state the facts clearly for the benefit of beaurocrats who may not have first-class English.