IF there was a prior will leaving you part of his estate and the new will changed or replaced it, you could challenge it, as you are an "interested party"--someone affected by the will. Of course, the will would only be set aside if you can show that he was not mentally competent when he signed it (such as due to medication), or was threatened/coerced or duped/tricked into signing, or that it was not his signature (e.g. she forged it), or that it was procedurally defective (didn't have enough witnesses or wasn't properly notarized). Otherwise, if the will was properly created, it will be valid and enforceable, even if it is unfair.If there was no prior will, however (or there was an earlier will, but you did not inherit under it, either), you can't challenge it. "Fiance" is not a legal relationship: it gives you no legal rights. A fiance does not inherit when there is no will; therefore, if there was no prior will naming you, you would not have inherited anything, anyway. That means that this will did not change anything for you or harm you in any way, and so you are not an interested party (not a person with anything at stake) and cannot bring a legal action to challenge the will.