"have a nice life" means: Said by one person to another, signaling that their relationship is irreparably damaged to the extent that the speaker no longer wants to be.... This is a fairly neutral word with no inherent risk attached. There is no real cause for parental concern; it is descriptive vocabulary rather than something dangerous. If your child uses it, context will usually make the meaning clear. A brief, curious question about where they heard it is generally enough to know whether to follow up.