MacBook Pro wont boot at all (El capitan)
This definitely sounds like hardware issue, based on the fact that the HDD worked on a similar Macbook Pro.

Before we rule it as hardware, lets perform an Apple Hardware Test .


  Apple Hardware Test (AHT) contains a suite of diagnostics that will test the hardware of your computer. It's a great way to rule out a hardware issue when troubleshooting your computer.


If your Mac was released after June 2013, use Apple Hardware Test.

Otherwise, if your Mac was released prior to June 2013, use the Apple Diagnostics Test.

Fortunately Apple does a great job with documentation, so I'll avoid repeating the instructions mentioned in the provided links.

Once the test is complete, please reply with any error codes presented.