could be, when you have a number that many or more than 50% then all the things you can do include control bitcoin. I'm sure, that there are currently doing so.
To be honest it is really hard to understand what are you talking about, you mean if the person owns 50 per cent of all bitcoins? Because it is impossible... And it is impossible to manipulate bitcoins for sure, the price depends on all the people.
All manipulations that may even occur with bitcoin, do not go to the benefit of cryptocurrency. Many users do not want to risk it and go, and thus suffers from the popularity and spread of bitcoin.