I remember that one. I think that was one of the first Hardy Boys books I read, so it formed rather a good bit of my impression of the series. I did not choose to read further.

Also it formed a good bit of my mental reference for the "fridge the girlfriend to drive the plot" trope, and my distaste for it.

The actual explanation for that one was particularly horrific (especially for the not-Iola woman involved) and also felt very derivative even though it was my first exposure to that particular plot.