I think in your situation, Singapore is the best bet. Immediately, try having United change your mother's flight to SFO-NRT-SIN or SFO-HKG-SIN. It may be too far in advance for a waiver to apply, but I wouldn't wait around, so if they want a change fee but it is minimal (say $150-ish), I'd do it. Then have your return flight changed to match hers. I would then buy a separate one way ticket BKK-SIN, there should be plenty of deals around ex-BKK on either mainline or cheaply on AirAsia.
From your meeting point in Singapore, you can travel into peninsular Malaysia, or get relatively inexpensive flights over to east Malaysia (Sarawak and/or Sabah) and do that. Or to a place like Bali. You can still get to Cambodia or Vietnam if those are priorities. Plenty of options if you want to avoid Thailand.