I think you have made your QA engineer work on the right thing at the moment. As you said the project is going through extensive changes, I would suggest you put your QA engineer to handle tasks related to QA services like:
-Exploring the project requirements and goals
-Writing test policies
-Define the test process based on existing requirements and ...
Lay a foundation for quality
"For the moment, we are asking the QA engineer to go through customer business requirements and to validate the existing functionality against them as well as familiarize himself with the roadmap."
That is good for a few key scenarios. However, beyond that it may lead to a testing setup that looks more like an ice ...