Respectfully, I disagree. Things that you should learn over 7 years of experience go far beyond learning the language itself or the fundamentals of its standard library. We're talking about two very different ecosystems.
Are we talking about the vast differences between Akka and Akka.NET? ;-)
Honestly, to the extent the ecosystems are different, it is probably more valuable to add someone to a team with experience with a different ecosystem unless there is literally no one on the team with experience with the current one (and in that case, you might question your commitment). Yes, it takes a while to learn a given ecosystem thoroughly, but that's not really what is going to drive the productivity of your team.
Would I hire a bright candidate that require retraining given absence of those that do not? Quite likely yes.
Would I be pissed at false advertising? Again, yes.
While it was extremely unethical for him to change your CV, Java and C# are indeed very similar, to the point that most organizations use devs with that background interchangeably.
If that company really ruled you out because you had 7 years of C# experience, and not 7 years of Java experience, you likely dodged a bullet.