Ah, I see your error. Somebody must have lied to you about what the Church teaches.
Whether or not a lie is a mortal sin depends upon the nature and seriousness of the lie. For example:
Mortal sin: If you tell us vair ze Judan are, ve vill not harm zem.
Venial sin: No Mommy, I didn’t take a piece of candy.
No sin: We have no idea where the Anne Frank family is.
Of course, the Born Again preacher contends all three are serious sins, as serious as rape or murder.
Go figure!
Does it break the commandment against lying?
I think the context is missing here. Most likely, the Born Again preacher is contending that ALL sin is "mortal" in the sense that sin means "missing the mark" and the "wages of sin is death (Romans 6:23) and "mortal" means "subject to death". It is about our inability to BE sinless and holy on our own. Scripture tells us that even a LIE will prohibit a soul from entering heaven (Revelation 21:27)). Yes, there are levels of sin - some more evil than others - but we are ALL sinners deserving of death but saved by the grace of God.