you have a few choices. Go to Thai immigration and apply for an extension. Or go to a visa agent and they will do for you. You can do a border bounce which will take longer than getting an extension at immigration. Or you do nothing and leave Thailand after your 35 days. You will be fined for each overstay and you will have an overstay stamp in your passport. This is not good if you plan to return to Thailand in the future
A multi entry is a stamp you get in your passport when you have a retirement visa. The cost is 3800 THB and is valid as long a your retirement visa is valid. You can also purchase a single entry which is 1000 THB. Both you purchase at immigration.
I still think it is better to use a visa agent the first time. Then do the renewal every year by yourself. Yes I know it will cost you a lot more than doing it yourself but no hassles. And you wouldn’t be writing in this form.
Absolutely not She would either apply for a visa from the Thai embassy in the country where she lives or apply for a retirement visa if she meets the criteria. I am assuming that is the course she wants to proceed with.