Can a 为什么 question be rhetorical in Chinese?
你为什么要浪费钱？standalone doesn't look it, whilst 你为什么浪费钱？does.
你为什么浪费钱？can be a blame or scolding, implying that there is no reason for you to waste money hence it's a rhetoric.
Also, Tone and context play important roles.  Your original sentence might be a rhetorical question if the proper tone is applied in a proper context.
However, the native speaker you asked has some point there.  为什么 is kind of on the borderline.  You can interpret both ways sometimes.
It's worth to mention that 为什么不xxx  is a common rhetorical device.  E. g.

A: 你要吃吗？
B: 为什么不呢？


A: 你会去吗？
B: 为什么不去呢？