Legal is really not the term used in estate practice although I certainly understand what your concerns are. The Will would not be valid. What is going to happen is that the Will is going to have to be contested once the person passes away and then it must be contested. Or you can move to set it aside now based upon the fact that the person was not of sound mind when it was executed.  It could be easier to do the latter as you can bring in the person and their doctors during a hearing.  Get legal help. Good luck.