Should he learn 8051 first then embedded C or embedded C first then 8051 or should he learn both simultaneously.

If you only know "C on the PC" (which, in the worst case, would mean "C on Windows", which is pretty much as removed from the hardware as you can get with C), then learning some general things about the inner workings of a CPU would be a good start. Things like:

- What are registers, and what are various types of registers (index/general/accumulator/etc) used for ?
- What are addressing modes ?
- What are Harvard and von Neumann architectures ?
- What are interrupts ?

to name a few. "PC programming" nowadays involves only minimal contact with the actual hardware (it used to be different), while uC programming is pretty much all about dealing with the actual hardware.

From there, learn about the particularities of the 8051 architecture (different memory types, bit manipulation, SFRs, register banks, the interrupt system, etc).

Then, learn 8051 assembly. At least enough to know what instructions and addressing modes exist.

Then, learn "C on the 8051".