Hi
There are steep edges near the path when it is crossing the plateau - and if the mist comes down careful navigation is needed as this link
http://ben-nevis.com/images/navigation/nevis_aerial.jpg shows.
Given the length of the day, as someone whose father had dementia I would be concerned you were putting yourself in a difficult position, albeit for good motives. However I guess everyone is different and your father may be less affected than mine was.
Whatever you decide it may be worth having a plan B. The Ben is reputedly only cloud free one day in six, on average and I have spent a number of visits to Fort William in Nevissport cafe as the rain ran down the windows and the Ben was clagged in before i got a clear day.
Would you have time to see how things go on a less committing hill first? Even one such as Aonach Mor, where you could take a gondola down if you need to.
Good luck, whatever you do.