The “best” topper would probably depend on why she had the back ache and also as monkey_donut pointed out in very similar circumstances … on what her height/weight and especially preferences were. I’m guessing the mattress is too firm for her side sleeping? … which probably leads to her “twisting” either the upper or lower body and spine to avoid the pressure points or to not sinking in evenly enough into the mattress and sleeping out of alignment. Depending on the reason for the back ache, a suitable pillow can also make a difference but I would suspect that the biggest “culprit” in your case is a mattress that is too firm for her.
Your mattress as you know is synthetic Dunlop and in combination with it’s thinness would lead to being very firm which makes for a good support system for back or stomach sleepers but for most people it wouldn’t be soft enough for good pressure relief for side sleeping.
Since you like the “feel” of latex … it would make a very good choice (I would suggest either natural Dunlop or blended Talalay as having the best value) but while it is very durable and has great qualities and in the right thickness and softness can provide excellent pressure relief, … it is also more expensive than other materials.
So your “best” option would be the thickness and the firmness level that allows for enough pressure relief for your wife but doesn’t compromise your support and keeps you as close to the support layer as possible.
As monkey-donut pointed out (and thank you :)) … there can be a big difference between “theory” based on averages and someone’s personal preferences so your own testing on mattresses with a firm support system and different types and thicknesses of latex comfort layers would be a more accurate guideline than long distance suggestions but I would think that you would need at least 2" for good pressure relief with this mattress and possibly more depending again on your heights, weights, body shapes, and preferences.
Memory foam can also be a good choice and is also very pressure relieving but has a very different feel so the choice would be based entirely on your own preferences rather than one being “better” than another. If you go in the direction of memory foam, then a more breathable faster responding type would be cooler and may also feel better for you than the slower more temperature sensitive varieties. Higher density memory foam (5 lbs and up) tends to be a little warmer and slower responding than lower densities but there is a wide variety of different memory foams in every density level and a higher density will also be more durable than lower density but will also be more expensive. I would also make sure that any memory foam you buy is either CertiPur certified or failing that at the very least made by a foam manufacturer that is well known for good quality foam (preferably North american).
Latex or even polyfoam as a topper would likely be “safer” if you’re not sure how you feel about memory foam although there are lots of outlets around where you could test various different types and layer thicknesses of memory foam to see how you feel about it.
This thread has some good choices for inexpensive memory foam and polyfoam toppers.
Some of our online members (see post #21 here) sell high quality latex toppers at good prices.
So to recap, local testing to help you narrow down what you need and prefer and would give you a helpful guideline for either an online or local toppper purchase. Local foam shops that carry different thicknesses and firmness levels of latex can also be helpful. If you are unsure, then an online outlet with a good exchange or return policy (which is not the “norm” for toppers) can help lower the “risk” of buying a topper online where the choices and recommendations would be based more on averages than on your own needs and preferences.