It's normal for moms to worry about kids who've left the nest and are on their own far away. (Source: personal experience with kids living far away for university.) If LW's niece were just starting grad school in another country, a daily check-up text for the first month or two may be appropriate to make sure she's settling in. A recent emergency or serious illness on either end also may warrant more frequent contact for awhile until the crisis passes.

Except LW's niece is 28, has lived in the other country for some time already, and she seems to be happy and thriving. The niece's preferred frequency of 2-3 times a week is imho reasonable. If everything's otherwise going okay on the mother's end too, then imho 2-3 texts/week should suffice. The mother may need to address her separation anxiety with a therapist. Otherwise, insisting on daily updates might result in no contact at all.