As I am not licensed in OR, I can only opine with general information as to the legal concepts surrounding "undue influence." With decedent's estates, when one questions undue influence of the testator, they are questioning whether the testator was so overcome to external pressures that he/she was unable to exercise sound judgment in the authoring of the will and/or in the disposition of his/her property. Introducing evidence/testimony regarding the event which resulted in your father's could conceivably impact the validitiy of the will. If your father was placed in danger by this person, feared retaliation by this person or was continually lied to regarding you and your other family members, facts along those lines could be significant. You comment regarding the person's financial status makes your concern about undue influence all the more a relevant inquiry. Also, while you do not address it in your question, it would be beneficial to know if your father was suffering from any dimished capacity or faculties (i.e. dementia, unable to speak, etc) that could have rendered him more vulnerable to what this person allegedly did.