There are many ready-to-eat packs (or gravy packs) which may be best for initial trial to see if Roro-san likes the taste.

The one closest to sambar will be this (it's kinda like sambar + rice)

https://www.bigbasket.com/pd/260767/mtr-masala-bisibelebath-powder-100-g-pouch

But I am not sure how easy they're to get in Japan