I didn't say anything about challenging the will on the basis of him not having testamentary capacity? I hadn't heard he had a trust, just saw it mentioned in this thread as a possibility. Even if he did that doesn't necessarily carry the day.
Interestingly enough a lot of people wo create trusts during estate planning don't follow through to actually fund the trust, which is necessary. Sometimes that's handled by what's called a pour over will, wherein assets that become part of the estate upon death are then automatically deeded to the trust via instructions in the will. It's not all that unusual for the trust to be unfunded, the will silent regarding deeding assets to the trust upon death and the estate being distributed via terms of the will that may not have been envisioned to control and distribute all assets. Or if the trust was simply left unfunded prior to death and there is no accompanying will, the estate is distributed according to operation of law (distributed to next of kin as recognized by law.)
A few years ago my mother actually was contacted by a law firm in a case like this. She inherited a few hundred dollars from the estate of a very distant cousin of her mothers she had never even met who had lived across the country. It wasn't until my mom received the questionnaire seeking family info. that my mom understood who this relative actually was according to other family names listed. Apparently she had married an incredibly wealthy man and they had no children. She had lived several decades after he passed and had no immediate relatives left. She left no estate planning whatsoever so the law firm that had been representing her and her husband long before he passed ended up being appointed to locate her heirs under operation of law. Her first cousins, my grandmother's generation, had all passed, so that meant they had to make extensive efforts to locate the next generation of cousins who were now deemed closest heirs in the stead of their parents. Several of the first cousins had seven to nine children each so in the end the very significant assets wound up largely eaten up by costs and then distributed to many, many heirs, resulting in very little of the $$ actually being available to distribute to "heirs". Apparently investigation and settlement of the estate took well over two years.