Hi ejcrossl,
I don’t know for sure of course but if she is sleeping better on a thin Ikea foam mattress I would also guess that the mattress is too soft rather than too firm. If this is the case then some of the suggestions in post #2 here may be helpful.
The first thing that I would check and rule out as a possible cause of any issues she is having is the support system you have under the mattress and the effect of any mattress protector or mattress pad you are using on top of the mattress.
Phoenix