Maybe you can help us by telling us if she has certain pieces, and does she prefer YG, WG, RG, Platinum, or Silver?
If I had to only wear one of my necklaces forever and ever, I'd go with my VCA all gold astrologie pendant on a 4mm woven gold chain.
My Aunt Charlotte wore a medium thickness gold chain with a small round pendant that had a diamond in the center on one side and an etching of her husband's photograph on the other, with another chain layered. She wore this every day for like 55 years.
VCA VA diamond pendant was a great suggestion from someone, too.
My MIL wears a mid chest length 4mm gold heavy link chain with a 1.5" or so St. Christopher medal in heavy yellow gold. She's worn it for probably 65 years or so now....