Point out that the passages they quote do not prove their case, but can be interpreted in other ways. Isaiah 26:14 describes Israel's defeated conquerors as "shades that cannot rise." This means they are unable to bring themselves back from the dead. Isaiah 43:17 and Jeremiah 51:57 refer to the dead's inability to get up physically; in the case of Isaiah 43:17 it is the inability to get up from falling down, and in the case of Jeremiah 51:57 it is the inability to get up from sleep. All three passages are qualified by their time frame, which is limited to this age and does not have the end of the world in view. It is within this age that the dead will never rise and will always sleep. The end of time is a different matter.
When we turn to those passages where the end of the world is in view, we see that the wicked will be raised on the last day. In John 5:28-29 Jesus tells us, "Do not marvel at this, for the hour is coming when all who are in the tombs will hear his voice and come forth, those who have done good, to the resurrection of life, and those who have done evil, to the resurrection of judgment." We are told that all the dead will hear his voice and arise and that the wicked will experience "the resurrection of judgment."
In Revelation 20:12-15 we read,
And I saw the dead, great and small, standing before the throne, and books were opened. Also another book was opened, which is the book of life. And the dead were judged by what was written in the books, by what they had done. And the sea gave up the dead in it, death and hades gave up the dead in them, and all were judged by what they had done. . . . [A]nd if any one's name was not found written in the book of life, he was thrown into the lake of fire.
Here we are told that all of the dead, great and small, will be judged by what they have done. The sea, death, and hades will give up the dead that are in them, which means none of the dead will be left unresurrected. Among the resurrected will be the wicked, who will be damned.
The Christadelphians' argument from 1 Corinthians 15:18 is flawed. Paul states that if Christ is not raised, then those who have died in him have perished. The simplest way to refute this is to turn it on its head: Christ was raised, therefore those who have fallen asleep in him have not perished--they are still awake and conscious with him in heaven.
Further points should be made:
First, for Jews the alternative models of the afterlife were total annihilation (this was the Sadducees' view) and resurrection (the view of the Pharisees). When Paul says, "If there is no resurrection then the dead in Christ have perished," he may be alluding to the Sadducee view that there is no survival beyond death. He is not thinking about a disembodied existence because, in Jewish thought, a disembodied existence is a just temporary state preceding the resurrection. If there were no resurrection, there could be no disembodied state either. (On the fact that there is a conscious, disembodied state, see Lk 16:19-31 and Rv 6:9).
Second, your Christadelphian friends have assumed that in 1 Corinthians 15:18 "perished" means "been annihilated" or "ceased to exist." This is not necessarily the case. For example, in Ephesians 2:1 Paul refers to a spiritual death (being "dead in one's sins") that can be experienced even while one is alive. His point in 1 Corinthians 15:18 might thus be that those who have died as Christians are not only physically dead, but spiritually dead also if there is no resurrection; they pinned their hopes on Christ in vain. This is the thought of the previous verse: "If Christ has not been raised, your faith is futile and you are still in your sins" (1 Cor 15:17).