If you are subscribing to something and the subscription price changes, that's not "lying", that's one party renegotiating. Furthermore, Hetzner's legal agreement allows them to do this.
Sure, Hetzner could publish "up to X TB / month" (and it sounds like it should considering their apparent audience). But let's have a little empathy here, sheesh.
Whether or not something is technically allowed in the fine print is different from whether or not it's a shitty thing to do. Having secret, unpublished limits on a supposedly-unlimited service is shitty. They'll be getting no empathy from me.
No, just as you said with your underpaying hypothetical, the customer is free to drop Hetzner. A rolling term contract protects both parties. If this person pushing 250TB/mo can find a better provider, then they should move.
If they can't find a provider willing to offer cheaper bandwidth than Hetzner...well, that tells you everything you need to know about this situation.
Pull the other one.