Is there grief counseling available for my mother with early dementia who just lost her spouse? Over the phone with Covid?

Speak to her doctor about it, that would be your best bet. Depending on her ability to comprehend information, she may or may not benefit from such counseling.

Good luck!

What are your indications that family intervention is not enough and that grief counseling is required?

This will be hard depending on how advanced her Dementia is. She may not be able to comprehend what she is hearing. Nor can she express how she is feeling.

I'm not sure about over the phone, but pretty much all GriefShare meetings are now done on Zoom, due to Covid. If she had hospice involved for her husband, she can ask them, as they do offer counseling.