my friend used to charge it up while I was away but she tried today and it was not working....

what options do I have? Can I say goodbye to number? If I call the number it says the number does not exist....  my friend used to put 20 bht every few months to keep the number active

I have the same set up. I pay 10 baht a month. Like you, I don't live currently in Thailand but need to keep my phone number active. She needs to go to the main office in Bangkok. I belive it is on the 2nd or 3rd floor. Great service and they should be able to  get things straighten out.

I would have your friend stay with 10 baht a month and pay it on time. Just my 2 cents.

Hope it gets cleared out for you. Most malls have AIS service centers but I found them useless to my needs or ... they didn't know what to do.

I pay 1800 baht once a year

You may be able to call them to reactivate it, or don't bother and buy a new sim. If you had a thai bank account you can top up easy enough