When they say feed sparingly, they are referring to the fruit part, what we think of as the vegetable.
Bush okra, or jute mallow, is a close relative but what your friend is growing is surely what we think of as “regular” okra here in the U.S. Like this, but I imagine the plants are much larger in your area this time of year:
It is in the hibiscus/mallow family, as you noted. Safe to feed in moderation. I’m growing some for my tortoise right now! I’ll feed the flowers as well, in lower quantity.