That will depend on his mental state at the exact moment he signed.
Competence can get a bit complicated and messy; but just because a person is "coherent" does not necessarily mean they were legally "competent". So whether he was competent will depend on exactly what he was signing, if he knew what he was signing, what rights he was giving up and to whom. So there is no "clear" answer without getting into the fine and minute details.
I'd contact an attorney and get into depth over what happened.
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Legal competence to create a will is a low threshold. A person can be legally competent to create a will if they have knowledge of who their family is, and a knowledge of what they have to give away. Just those two things. Pain medications can bring competence into question, but if doctors say that someone was coherent, that will generally indicate competence.
There is another question that may affect the validity of the will, and that is undue influence. If someone else was influencing a person (by force or threat) to put certain things in their will, that can invalidate the will.
Both incompetence and undue influence are hard to prove. If you think one of them occurred, you would have the burden to prove they occurred. this is very hard to do.
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I agree with the other responses and particularly with Attorney Krause. We had a situation where a woman was in the ER, in shock. She was legally blind and did not have her glasses. She was legally deaf and did not have her hearing aids. She was intubated and could not speak. The lawyer who came in to the hospital said that she "read the Will" to the woman and communicated by squeezing her hand. The Will was littered with typos, including reference to a husband, and this woman was not married. So presumably, if the Will had been READ, there would have been a lot of hand-squeezing. The Will was signed either right before morphine was given or right after.
Result: valid Will.
So you can see, these kinds of cases are tough. To have any chance of overturning this, you will need a very skilled probate litigation attorney. Best of luck to you!
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