You write in English. She reads it in Thai on her own phone, in the app she already uses, and her answer comes back to you in English a few seconds later. Nothing reads like a phrasebook and nothing gets lost on the way over.
She joined by choice and can stop whenever she likes. You can buy her a drink at the bar she works at from anywhere in the world, and once you have been talking a while you can ask her to video call, which she starts from her side.
Your first messages are free and there is no card to start. After that a pass is ฿299 and it covers every conversation you have here. The longer version, if you want to know exactly what happens to a message.