How can God allow anyone to go to hell?

A friend asked me this question the other day: "How can a Loving God and a Horrible Hell both exist?" This problem is not new. It implies that if God really exists and God is so loving, how can he allow anyone to go to hell.

How can a loving God send anybody to hell?
http://www.christiananswers.net/q-grace/hell-and-god.html
Does God really send you to hell? or is it the man in the mirror? God does not desire anyone to go to Hell. He weeps when it happens.

Is the "burning fire" in Hell literal or figurative?
http://www.christiananswers.net/q-grace/hell-fire.html
Ans: Does it matter?

Does God feel our pain?
http://www.christiananswers.net/q-aiia/god-pain.html