Our axies lived with gravel and never, to my knowledge, ate it. This is a new idea to me and I wasn't aware they did this (we had them when the kids, now adults, were small).
If you use sand you can't use an undergravel filter (ugf).
A small internal canister would work but take up room in the tank. Check Trademe for a small external canister with a spray bar return so the current is spread out a bit or, like Sharn suggested, don't have a filter at all. I assume she does very regular water changes too if there is not filter.