When someone is grieving, they must be allowed to grieve completely - no matter how long it takes. It is not up to you or anyone to determine that they have mourned enough.
By no means, never suggest that they get over it and move on. Never suggest platitudes like, "Time heals all wounds."
Those words will not help them even though your intentions may be sincere.
The mind will do what it does best and develop its own coping mechanisms to help restore mental balance. The Soul will observe the mourning process of Its body until It is ready to move on to the next phase of experience.
Your inability to handle or accept someone's prolonged grief will not alter a single thing, except within your own mind.
But do not worry - grief is healthy, and as long as grief is allowed to present itself unconditionally, the process will allow the Soul to more easily guide Itself along a more fulfilled path in the future without psychological occlusion.