Since Nico can't see/feel his mana (as shown by him getting upset he's feeling bad but doesn't know why) I think he and anyone who doesn't know how to control mana wouldn't be able to tell if they are running out or not, so the magic stone would just keep absorbing their whole mana pool.
Nico didn't seem to understand that Lia was absorbing his mana, he just noticed she was getting warmer.
Of course, but in this instance, Lia could have been the one to see the mana for Nico. She could have waited for her dad to get magic stones, for Nico to touch them, and then have him stop when it looks like he's fine again. I did not expect that she would try to directly extract Nico's mana surplus - she got impatient about helping Nico.
Nico is also a special case; I mean, when Lia got rid of the mana she extracted, it took two decently sized small stones. Also, if I interpret page 15 correctly, he is far from being drained and still has a large reserve of mana.
The main thing for me though is page 8 - Nico might not know how to control his mana, but he definitely can tell when the bad feeling from having too much mana is gone, so if he pays attention to that feeling he should know when to let go (and he does not need to be too precise because it literally takes several stones to remove his surplus mana; I expect he could easily fill a third stone and be nowhere near collapsing).
I understand that all these elements are things that Lia only learned after extracting Nico's mana and outputting it in stones, but having people input mana in magic stones and then slapping them when they're about to run out is something she's done before in the frontier.
But there seems to be a trick to filling one, maybe it sucks too fast and if you're not good at managing your own mana it also dries you entirely
Yeah, something like that would definitely explain it, but I'm not sure she would know about this. Maybe somethig like that was mentionned previously and I forgot?
Also, Lia keeps doing things that she hasn't really done before (like taking in someone else's mana) and not putting any thoughts as to whether there might be any dangers to it.