Bella33 - I think you're on the right track. I take Centrum Silver, Super B Complex, and B12 daily. I'm 75, and find it makes a big difference in how I'm feeling, compared to when I stop for a few days, as I occassionaly do, just because I get so tired of the hassle of having to crush them, and then how yuk they taste, even mixed with applesauce.
When I was caring for my mom, I gave her a multivitamin, and noticed a difference in her fairly quickly. My problem with that was it brought out the side of her that would have made my staying with her impossible, so I discontinued them and just kept her active, physically and mentally, which made her happy again, and also kept her in a lot better shape overall than she'd been in when I'd gotten there.
An added B12 daily might also help your mother. Its deficency symptoms include appitite loss, diminished reflex response, memory impairment, mental depression and confusion. You might try it for a short time to see if it helps more. I've never seen any warnings of B12 being dangerous, but you could check that out first.