Deep emptiness (nothingness) while meditating
Provided that your mom is well and healthy, your description fits with the buddhist perception of emptiness (sunyata). It's not dangerous, and the experiences could be the result of a progressed concentration thanks to meditation.

In short, sunyata can be described as a state during meditation - just like you described it - but it also has connections to a way of perceiving the world, in a general sense.

Below is a link that discusses the phenomenon, as well as referring to relevant suttas:

https://www.accesstoinsight.org/tipitaka/mn/mn.122.than.html

(Perhaps an online translation service can help to display this content in sinhala)

Lastly, an unsolicited advice would be to consult a buddhist monastery, where you might be able to recieve some more substantial guidance from experienced practicioners.