I was mining with someone i met in sg today @trefjc and he didn't die when he fell in lava. Not sure why this occurred. I later tried going in lava to test it and i was taking damage
If you really believe that hacking did happen, then file an actual complaint. Before that though, have you ruled out the possibility that the user was not using a potion, or that they do not have pyro?
First idea: www.ecocitycraft.com/forum/threads/127413 Further support of that idea: I managed to reproduce this bug in the mining world once (although I cannot find the forum post about it). Either relogging or leaving and re-entering the Mining world should fix it.
I am certain that the health calculation (in this case, EntityDamageEvent.DamageCause LAVA) is processed by the server. A client should never be the only one responsible for something as important as this, it would make exploiting infinite health and invulnerability far too easy.