Hell, no matter what it's other qualities may or may not be, is eternal separation from God with no hope for redemption. While on this earth, yes, we are separated from God because of sin, but we still can accept Christ and be forgiven of all sin, which lifts the separation. Even so, God still gives us opportunities to accept Christ, so even in this separation we are not in Hell.
However if we die without accepting this truth, it is as if we permanently rejected the truth, no matter what the choice was. Indecisiveness or putting off a decision is the same as rejection. So if we die with this rejection, we are eternally separated from God. No matter how much we may or may not beg for forgiveness at that point, it's already too late.
As far as Earth being Hell, that's obviously not true. I don't know what you can say to them to convince them otherwise, but I hope I helped at least a little bit with what I think Hell is. I'm not sure if there's fire or not before the final judgment in Revelation regarding the lake of fire, and I really don't want to speculate on it. I just know that Hell is the eternal separation of us from God with no hope for redemption.