IANAL but understand that the test for Testamentary Capacity is that the testator must:-
● understand the nature of the will and its effect
● have some idea of the extent of the estate they are disposing under the will
● be aware of the persons for whom the testator would usually be expected to provide (even if they choose not to) and be free from any delusion of the mind that would cause them reason not to benefit those people.
It would be wise for the will to be witnessed by a suitable medical professional.
Iin some cases beneficiaries may discuss the will in conjunction with the testator but if there is conflict between the beneficiaries and the capacity of the testator may be in doubt I would tread very carefully.
The beneficiary concerned could, after the death of the testator, change the terms of the will by Deed of Variation if the other beneficiaries agreed.