Avoid fried foods cooked in most vegetable oils other than olive oil, though.
They would be fine if total consumption of such vegetable oils is low, but all too commonly it is far too high already and so should not be added to.
(Besides olive oil, organic coconut oil, macadamia oil, and tea seed (camellia) oil are also fine.)
With regard to your mother using a mixture of olive oil and some other vegetable oil: Don't worry about it, as mentioned, eat your mother's food. Be glad that she has the olive oil in there.