It would seem it’s up to the Thai official. If he thinks you are abusing the 60 day visa exempt, they ask you to get the correct visa if you want to stay longer ( I may stand corrected but it’s how I understand it )
Do it yourself on website. Yoh need show she will return after visit , have letter from her employer , if she owns home prove it. Show you have enough money to support her, don’t waste money on agent
You need get non O visa from uk ( Thai embassy ) quite easy. Then when you get to Thai you can open bank account with that. Then apply for 12 month extension ( and again each 12 months ). I had to get a letter from immigration as well ( quite easy take 4 photos with you ). When you here if you want easy way to get 12 not n extension try thaivisacentre.com on bk they are brilliant.