How can I help my mother transfer her Non-O visa dependency to my Non-B visa after my parents separated?

Where is your mother's visa located and how old are you?

We will provide you with the correct advice.

A couple years ago, I applied a Non O-A visa for my husband from outside Thailand which was a mistake. When almost his visa expired, we went to the immigrations, but he couldn't extend another year because we couldn't afford the health insurances' signatures to certify for the coverage. However, the officer told us there's a way by flying (only fly not cross border/not visa run) out of Thailand and flying back to Thailand again (we did flew out to Singapore and came back to Thailand same day). Then he got a 30 days stamp and after 15 days of stamp (or 15 days before that 30 days stamp expire). He must apply for a Non-O 90 days visa. Then wait until one month before visa 90 days expire, he must apply for a Non-O 1 year visa. I helped him to do and everything just fine like the officer recommend. I thought your mother should be fine with the same situation as my husband. These are a little bit complicated but as long as it works. I thought you can ask the immigrations for sure but your mom must be over 50 yrs. (My husband is over 50 yrs.) The important thing this kind of Non-O i year visa must have 800,000 baht in bank account to show to them. But I'm not sure about Non-O on Non-B visa. Hope this info help.

How old is your mother? I think she needs to be over 50 to become your dependant. Assuming she is it and shes actually on an extension of stay it should be possible to cancel one extension of stay and immediately apply for a new one. But as Brandon says without the letter from the employer to cancel the existing extension its going to be difficult unless she can show she is no longer eligible ie they get divorced

Without the letter from the employer, there's very little that can be done.

Most likely she'll need to leave Thailand and start over with a brand new visa as you won't be able to cancel the existing extension without the documentation from the original sponsor.

It's probably best to get the new non-O visa from a Thai embassy before returning to Thailand if possible, as they may still demand cancellation of the previous extension even if it's already expired. Having a new visa may help avoid that.