Your partner’s conviction is an assumption, not a certainty based on the Bible. Hell was prepared for the Devil and his angels but, of course, it can and does accommodate many lost human beings. See Matthew 25:41; John 3:36; Revelation 20:11–15. Judgment ending in eternal damnation is a major theme in Scripture. The good news is that Christ Jesus came into the world to save sinners (1 Tim. 1:15–16). God alone knows how many millions or billions of sinners have escaped hell by believing in Jesus. They are saved, not by unfailing adherence to Christian standards of conduct, but by faith.
Suicide, viewed abstractly, is a terrible sin. It is repudiation of the grace of life. It is murder of self. But it is no more unforgivable than the last-minute doubting the goodness of God that some dying believers engage in. Many believers struggle terribly at the last hour. They are guilt-ridden and have no peace. Some are emotionally distraught to the point where they hasten their death by suicide. If they were genuine believers, they wake up in heaven.
Those who think that suicides go straight to hell misunderstand the basis of salvation. It does not depend on confession and the last rites, which are precluded by suicide; it depends on God’s honoring the promise made to penitents—that those who come to Him He will never cast out.