Can my Filipina girlfriend on a NON-O Volunteer Visa apply for a dependent visa if we get married without leaving Thailand?

Nope, she can't get an extension OR a visa based on being married to a foreigner who legally works here with that initial Non-O volunteer visa.

I'd talk to the foundation she volunteers for next time she goes there to volunteer and see if she can just get a 90 day extension when this current Non-O stamp runs out on the 7th of Nov.

She has to at least get ONE extension based on the reason the visa was issued, (which is for volunteering at that specific foundation)

AND

that's the reason I told you to have her talk to the foundation that gave her the paperwork so she could go to the immigration office and get the visa issued .