Except of course that no
formal request for an investigation has been made by the U.S. (suggesting that this is more of a smear tactic than a legitimate investigation into corruption).
https://www.thedailybeast.com/ukrai...-sonbut-only-if-theres-an-official-us-request
In addition you also have proportionality... Even if there is a legitimate reason to investigate Biden, making major foreign policy changes that can affect the geopolitical situation just because "Gosh, person X didn't pay their taxes" seems excessive (considering the Trump administration continues to support Saudi Arabia after they had a reporter actually
killed.)
And finally... lets say there was a reason to investigate Biden. Lets say the crimes were serious enough to suspend aid if they were uncooperative.... The Trump administration may have still broken the law by not passing on a legitimate report to congress in a timely manner, as they were required to do.
(This is of course assumes the whistleblower complaint was about Trump/The Ukraine/Biden, and not some unrelated reason.)