If you have no intentions to return to Thailand before May 2025, you just leave no need to advise immigration. If you do not have a re- entry permit your visa will be finished when you are stamped out of Thailand. But if you want to come back for any reason until May 2025 just get a re-entry permit before you leave that will keep your visa active until May 2025
Valid licence from your own country, a valid IDP, a certificate of residency from immigration, a doctor’s certificate, 2 passport photos, a copy of your information page on your passport, a copy of your last visa or extension in your passport, and some DLT offices you will need a appointment online. But always best to visit your local DLT office and ask them directly what documentation you will require.
If your friend is a passport holder of one off the 93 countries that are allowed visa exempt entry, she will get a 60 day visa exempt stamp which is free then during this time she can apply for a 30 day extension at her local immigration office which will cost her 1,900 baht, giving her a total of 90 days inside of Thailand.
Highly unlikely immigration at the airport will question her, if anything the airline she is using might question her upon check in.
It is not a visa, the proposed ETA is a electronic travel authorization, and it is only a proposal at this stage, so don’t be too worried, a lot of countries have a ETA in place.
Most immigration offices will give you a hand out which outlines the visa requirements for that particular office. Go there and ask them, it is a fairly easy process to do yourself, if you are organized and meet all of the requirements.