Why not tell him to start from Pozar's book "Micrwave Engineering". There is another one from Collin's "Foundations of Microwave Engineering" this is much better than Pozar's in my opinion even through I started with Pozar's book.
Another great book is Ludwig's & Bretchko's book: "RF Circuit Design: Theory and Applications" which has better depth in first steps.
there is a huge differnece between colin's book , and gonzales book
colin's book , ina therotical book covers many topics in microwave like Transmission lines , wave guides , propagations modes , peridic structures ,cavities
and so on
but gonzales book is concentrated in Amplifier design and oscillator
You might wanna begin with a fundamental communication circuit such as oscillator, mixer, freq synthesis, and rf amplifier.
I recommend an easy beginning book of
Smith, MODERN COMMUNICATION CIRCUITS (McGrawHill)
If you really want to get your feet wet on alot of the components and such,
the Chris Bowick book covers alot of topics with a fair amount of details.
It would prepare you for more advanced topics as well as introduce into most of the important concepts with enough depth.
Also it is not a large book
The other books listed are very good and each covers some important microwave
concepts. But I have found that, you need to look at alot of books to get a big picture and alot of the concepts down.
So in my opinion, in this order
Bowick
Pozar
Gonzalez
Then look for specialty books that describe 1 area, as you become more understanding of the harder concepts
There is no unique definition such "RF designer"..
Which area and which frequency range ?? Which type of circuits that you'll be involved to design ??
Obviously a radar engineering is not same as RFIC engineering or CATV designer can not involve into SoC design ..
There should be unique definition even for the basis.
But if you start from a good proper point into RF Engineering, I recommend you to read some basic books mentoned above but they are not sufficient..