Take her with you, and have her lay down on as many mattresses as it takes. Have her try memory foam, tufted, latex, spring....the works. Find out why the other one didn't work- too hard? Too soft? Too hot? Too cold? Did springs poke her? Mattresses are an investment, and since she's going to be spending 1/4-1/3 of every 24 hours n it, mattress shopping is something that's going to require her direct input.

Also remember that you're not required to buy from the place you're shopping at. Check out a few stores and get one that really works for her.

We had an old mattress when we moved out here, and my back had been killing me for months. We stopped at a mattress store, and we both lay down on each one. No dice. Three mattress stores later, I was getting incredibly frustrated and ready to give up, and I tried one last mattress. It was very firm, and the second I lay down, my back cracked all the way up. That was the sign!