Along the Vermont / Canada border there are farm parcels that cross between the two counties, and there are buildings that straddle the boundary. The town office building of Norton, VT is a former small grocery (aisle shelves still in place) which straddles the border. The town of Derby, VT has numerous buildings, including residences and an opera house that straddle the boundary. My great-grandmother was born in a house which straddles the border, and her birth certificate has a parenthetical note saying (US side of house).
This is just interesting fun. As to your issue, there is some legitimacy to the client asking for support that the two-state situation doesn't impact value. Support does not necessarily require sales data, though that would be the most convincing. The sale you do have could be compared to a group of non-straddle sales looking for differential. You could also find straddle properties that haven't sold in some time and survey the owners to see if they report that it affected their pricing, ask some brokers what they think, call a few appraisers. If you survey a bunch of participants who indicate they don't think its an issue, have a sale that is within comparable market parameters, and other wise have no data or rational reasoning the issue would affect value, then there's no support for the adjustment.
What would be the reasoning that would push value downward? Is utility affected, is the highest and best use affected? Is tax burden, transferability, or access affected? Is there a legal restriction, say building improvements that straddle the boundary are prohibited, that encumbers the property? If there is no discernable differential that affects the highest and best use, do you even need to support not making an adjustment?
That the portion in the other state isn't recognized for tax purposes doesn't mean it isn't in the other state. Whatever the legal situation as far as taxation of cross-boundary properties is controls. You should be certain what that is, whether by agreements or laws. Report the current tax situation, but remark if it's inconsistent with law. Also, note any differential land use regs, both state and local if any. Does the deed itself reference the property as two parcels or a single entity? Does the boundary create a de facto subdivision? Does the boundary affect future subdivision potential, if any, and does this affect value? Is the land in the other state necessary to support the current use? Is the highest and best use of the improved portion different if the portion in the other state were severed? Same consideration for the unimproved portion? If the answers to these questions are clear, then there really is no reason to analyze the adjustment.