I have considered this myself so have looked for information on line. There is info out there but FB is not really the best place to find it. It seems to be a difficult route but could be very rewarding if successful. I am married to a Thai who would make an wonderful mother but unfortunately we cannot have children. It may be less difficult if you are seeking to adopt a nephew or niece of your Thai spouse if that were what you are seeking to do. In any event if you do the ground work and follow the process then you could be successful.
In a nutshell no not as far as I know unless you pursue some other kind of visa like tier 2 or skilled worker route. But I am not an expert. My wife is on a spouse visa 5 year route to settlement which firstly gives 2.5 years then you need flr or further leave to remain then at 5 years ilr indefinite leave to remain then she can apply for citizenship. Other avenues may be a 10 year route. You could get more accurate detailed advice from an immigration lawyer in the UK or Thailand. Also it is an expensive process. You cannot convert a visitor visa you would need to make a new application. If I am wrong I am sure someone will correct me this is just from my personal experience.
You can apply for a spouse visa you just need to meet the requirements. You're welcome to pm me if you have more questions. I did it myself without an agent or immigration lawyer. First two visits on a visitor visa then a spouse visa.
I believe UK visa applications from Thailand are all processed in Delhi. Biometrics are done in Thailand. But where it is processed should not matter to the outcome. What matters is how the application is presented and the evidence that is uploaded. Pleased to hear your friend was successful in the end :)