You'd have to be committed to at least staying on for 5 years in Thailand to make learning Thai value for money & time expended as that money & time is very valuable for most people.
Do you even realize how many languages there are in the world? Thai doesn't even make it near the top 20 of most useful ones!
You should be able to get her passport back before then.
If you plan to cross a land border crossing to enter Cambodia she could telephone the border immigration at that crossing to check if it's possible to cross with a border pass instead of a passport. You can usually find the border checkpoint phone numbers from Google maps.