I'm not a caregiver, I'm a volunteer with an organization that has another volunteer with whom we are having problems. Any suggestions?

I would contact a family member explaining what you r noticing. It's been brought up on this forum not to exclude them from activities. Dementia gets worse when the person loses independence.

Is the volunteer a senior? Do you have an emergency contact as part of the application process? Would you be comfortable speaking with the volunteer or the emergency contact about the changes?
If dementia is setting in, even getting to your location may be more than she should do. We had a similar situation in one of my volunteer positions. The head of the group asked me for advice. When I heard what was going on I questioned the ability of the person to drive to the location. This got the group head nervous. Someone spoke with the volunteer about the work becoming too strenuous and asked if he would like to take some time off. He agreed and ... needless to say, hasn't returned. It is sad if it is dementia. Alternatively, perhaps just a change in the type of assignment may work.

I like the above advice. Maybe as you advise her that she take some time away, you could refer to her good work in the past (hopefully there has been some) and how appreciative the organization is of it, but that you (the organization) recognize that the well being of your volunteers is crucially important and it seems that right now active volunteering is interfering with that for her.

Approach her with compassion; obviously something is wrong.

Maybe something in her life has changed and is stressful, this can cause behaviour like you describe. Once you find out what is going on, it will help you decide how to move forward.

Offering her some time away from her volunteer responsibilities, so she can focus on whatever else is troubling her, is the least pushy way I can think of going about this.