Suggest you start by reading the uk government website on what is required. You will apply for a 6 month visitor visa multi entry, that is the only visitor visa available for your friend. I actually never showed flights as I only bought them once she had the visa so having flights is not required for the application. You will need to show many things as part of the application, I.e. she will need a letter from her employer saying she has a job, her salary and that she has been given leave and show a leave and return date, stamped with her employers id. She will need to show her passport, copies of her bank account, if she owns her house then copy of house documents in her name, these all help to show she has a reason to return. You will need to show your passport details, and invitation letter saying you will financially support her and what your itinerary is during her stay i.e. to visit the uk and give some indication of where you will go like London etc.etc. also state the address she will stay at, yiu will also need to show your bank statements so they know you can afford to cover her costs (£~
********
per week) and a bill with your name if she is staying at your house to show you have somewhere for her to stay, the letter needs to state she will not be a burden on the uk finances because you will cover it. Also she needs travel insurance which you can show/upload . You will apply in either chiang mai or bangkok through the VFS and have a conversation and biometrics as part of the application. It will take upto 3 weeks for approval. Hope this starts you going