You better have deep pockets. Some solicitor will get rich off this.
In Scots Law, spouses and offspring have legal rights and cannot be disinherited.
In England, and beyond those legal rights in Scotland, the general principle is that you can leave your worldly belongings to whoever the hell you like (cats home included).
Dependents may have a claim, but they would have to show that they were financially dependent on the deceased.
In law, vindictiveness is a perfectly acceptable reason to leave your estate to some random person in the phone book provided that you were compos mentis at the time you made the will.
Doctors should have records of the medication at the time (but bear in mind that someone who “comes and goes” can make a valid will during their good spots). The solicitor who drew up the will should also have records, if a solicitor was used. You also have the two witnesses.
It is much less about the time that has passed than it is about the records that exist. Generally, there is a presumption that the deceased knew what they were doing. If you have no or little evidence to the contrary then you will lose.
Your best bet may be to use a solicitor to fire a shot across the bows of the other beneficiary. Settling quickly is not a bad option for either party if you are committed to action.