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.
6 free messages to start, and more every morning at 9am Thai. No card to start. A pass is ฿299 if you want more than that, and it covers every conversation you have here. The longer version, if you want to know exactly what happens to a message.