"I wonder why they need a navigator in the crew, aren't their NH90 fitted with GPS?"
GPS can be jammed / denied / spoofed. Those that rely utterly on space-based support risk being non-effective if it is interfered with. Several units now practice "no space" days, with no GPS or SatCom used, just to make crews and mission planners think about that "worst day" scenario.
For a viewpoint on what space-denied might look like, read "Ghost Fleet".....
In the meantime, condolences to the family of the unfortunate casualty in this accident.