> If he knows C, he will know C++ too.

What?

The two languages are only barely related as used in industry. Tons of people use C every day who wouldn't think about touching C++.

  C++ is rarely used compared to straight C in:
  OSes 
  Micro-controllers 
  Libraries

Lots of users of C also think C++ is a horrid mistake for many application development efforts (hard to find engineers who do C++ right enough vs most other languages).

And those same C people have rarely programmed in GUI environments as well.