In English this would translate as 'My mother is following a diet', denoting that this is a continuous process. 'My mother follows a diet' is almost the same, but carries a subtle and ironic resonance, suggesting that the 'following of the diet' may not be that rigorous. This inference results from the fact that 'My mother follows..' is used much less commonly than 'is following...'.
In italian, present tense is used for both present and present continuous. unless something is happening right then in the moment This is different from english where present tense is usually used only to denote something habitual. Studio la storia. = I am studying history (at school) Sto studiando la storia. = I am studying history RIGHT NOW
Where are you from? In the states a general statement like this would be translated as "My mother is on a diet" (which is marked incorrectly by Duolingo btw). I've only ever heard "following a diet" when talking of some particular regimen, such as "she follows the Atkins diet", but even so, "she's on the Atkins diet" would be a more colloquial translation.