FYI, the OP clarified that the perp. lives in Missouri and that was where the abuse took place. If so, it appears that the SoL may continue until 10 years beyond the 18th birthday, or 3 years after the discovery of the physical or psychological injury caused by the abuse. She might consider speaking to law enforcement where this occurred.
Now, if she is in Texas and being harassed by her abuser, that might be a crime in TX, but I suspect it will be a misdemeanor and that seeking extradition from Mo. even if they do act on it would be unlikely. But, if she wants to try and scare him off or discourage him, maybe speaking to local law enforcement in TX would help. Who knows, maybe the Texas agency can ask for a Missouri counterpart to respond to rattle the creep's cage even if she does not want to report the crimes from her youth.
No matter her call on reporting it, I hope she is utilizing counseling resources to help her cope - especially in light of the contacts by the perpetrator.