For that kind of thing labradorite is the nr 1 go-to. Preferably one with yellow sheen as that works on the Solar Plexus which is where this enters the systems (hence the belly).
Would be good to wear it as pendant and low down, directly over the stomach (= Solar Plexus area) and on the skin, so under the clothes.
A crystal can't work miracles of course. It'd be handy if she would establish some boundaries. Depending on the reason why she doesn't, any crystal that works on the lower 3 chakras could be helpful.
Can't give more specifics without knowing the reason, and often there's an overlap in chakras anyways with what's behind it.
Personally I'm a big fan of yellow (opaque) calcite in such matters when you don't know the exact root cause but do wish to help.
Yellow calcite (NOT the optical one for this) is extremely gentle yet powerful and works on emotions. In many cases it works a treat and it has an effect on the Solar Plexus, Navel as well.
A nice somewhat larger piece, like a sphere or freeform for her living room, might prove helpful. It is gentle so not like you would notice instant huge difference. Yet, it does work.
It also has a joyful, sunny vibe :)
And I don't feel any obstruction in wearing the labradorite pendant and a yellow calcite in the room.
Nonetheless it is always good to ask the person. Show your mum a yellow calcite sphere or freeform or... or... and see how she reacts. Is it like "Wow, that's beautiful!" or "Ieewww.... horrible!"
If the latter then it's not for her.
You can do the same with a few other crystals. Maybe for her it's rose quartz, or red (opaque) calcite or or or ... Intuit and pick a few.
I somehow get green calcite now. The rough pieces, this variety --->
https://i.etsystatic.com/22533081/r/...53842_pb6l.jpg
That's rough green calcite with a wee bit of a honey calcite layer near the bottom. The energy of these is so loving and warm. Great room pieces! Google it to find out more and see if you think it's a fit for your mum.
Black tourmaline and the like isn't for changing anything but to absorb negativity. That still won't change a thing, it's carrying water to the sea. Plus, you got to cleanse black tourmaline regularly if it works hard as it won't rid itself of dense energy it absorbed. It absorbs, doesn't transmute...