I’m not sure I understand what you think he is going to be doing there. Him being part of the team that takes the engine apart does not mean that he will ever touch a wrench to the engine. He will be an observer and he will be asked questions.
Pratt & Whitney has a very strong interest in the outcome of this investigation. Especially as there is a fatality involved. If I was the airplane owner, I would pretty much insist that my mechanic was part of the team.
So no I see nothing inappropriate about him being part of the team.
The NTSB is supposed to be treating this as if it was a crime scene. Would you have any problem with one of the prime murder suspects (for example) being allowed to observe a criminal investigation, because he, his insurers, etc all have such a vested interest in the outcome?