If the person with which you wish to address this subject were not a Christian already, this would probably be a much different answer. Since you are talking with a believer, however, I will assume that this person believes in the authority of Scripture. Otherwise, you have a whole different argument on your hands about the inspiration of the Bible. All a person needs to do is to present the evidence from Scripture that Hell exists. It is up to your friend to accept God’s Word or reject it. In term of doing so lovingly, that is a matter of your heart’s attitude. If you share the truth with the motivation to help your brother, then you have done your part. If you enter the conversation being set on being proven correct for the sake of pride, beware. We cannot always control how we are perceived by others. Sometimes we are misjudged by no fault of our own. The response of your friend toward your defense of Biblical doctrine is not your responsibility. Your responsibility is to actually love your brother, not merely to appear loving. If you actually act in love, it should be noticed. Yet, again, the other’s response is not your responsibility.
If you are not familiar with the Biblical teaching on Hell, there are many references to it. Here are just a few: Jesus warned about it (Matthew 10:28, 23:33, Luke 16:19-31). Peter taught that angels and men go there (2 Peter 2:4, 9). John taught that Hell is a place of eternal torment into which both Satan and evil men are to dwell forever (Revelation 20:10, 15). There is no Biblical evidence that all people in Hell are punished equally. All unbelievers are to be sent to the “lake of fire,” (Revelation 20:15) but many interpreters understand the fire to be allegorical. The everlasting nature of the torment, on the other hand, should not be interpreted that way. If eternal life is actually forever, then so is eternal torment. Fire or no fire, what makes Hell so bad for those who are there is primarily their separation from God (2 Thessalonians 1:7-9).
If your friend’s concern is how a loving God can send people to Hell, it should be understood that God does not desire that anyone go there (1 Timothy 2:4). Because He is loving, however, He has given us freedom. To those people, who by their rebellion against God and their unwillingness to put their faith in His saving grace chose to be separated from Him, God gives what they have willed. He allows them to be eternally separated from Him. He will not annihilate their souls because of their choice of separation. That would be an attack on His own image as well as on the free will of man, which He has ordained. For more information on the doctrine of Hell, see http://carm.org/hell.