I would say that a person who denies the existence of hell could certainly be a truly regenerate person. All kinds of Christians have all kinds of theological weaknesses and errors. We always have to struggle with this. So, I don’t just make the automatic assumption that somebody is not a Christian if they neglect a particular doctrine.
That being said, to deny the reality of hell in any significant way certainly raises the question of whether or not a person is in the faith because it is such a central, core teaching of Scripture.
Jesus taught more about hell than He did about heaven. If you’re not willing to listen to Jesus’ teaching about the nature of hell or the existence of hell, then that raises real questions about how open you really are to your Lord.
I still think it’s possible that a person could be that weak in their understanding and still be a redeemed person, but it’s hard to imagine.