Friesen v Friesen Estate (1985) 24 ETR 191 sets out inter alia the heavy duty upon a lawyer taking instructions for a will to investigate and satisfy him or herself that the testator has mental capacity.
77 The law reports of England and Canada are replete with lengthy decisions setting forth the principles to be applied when testamentary capacity has been challenged, and describing the standards expected of a solicitor who has drafted a challenged will. Banks v. Goodfellow (1870), L.R. 5 Q.B. 549; Tyrrell v. Painton,  P. 151 (C.A.); Menzies v. White (1892), 9 Gr. 574; Murphy v. Lamphier (1914), 31 O.L.R. 287 (affirmed 32 O.L.R. 19); Leger v. Poirier,  S.C.R. 152,  3 D.L.R. 1 (S.C.C.), and Slater v. Chitrenky,  4 W.W.R. 421, 10 E.T.R. 191, (sub nom. Re Campbell; Slater v. Chitrenky) 28 A.R. 54 (Alta. Surr. Ct.) [affirmed  3 W.W.R. 575, 11 E.T.R. 171 (C.A.)], are only a few. Rather than review or quote extensively from those cases, I will enumerate what I regard to be the basic rules to be garnered therefrom, as they apply to this case.
1. Proving testamentary capacity rests upon he who propounds the will or seeks to take advantage therefrom.
2. For a testator to be of a sound and disposing mind, he must understand the extent of the property of which he is disposing; he must be able to comprehend and appreciate the nature of the claims of others who might be expected to participate in his bounty.
3. Whenever a will is prepared and executed in circumstances which arouse the suspicion of the Court, it will not be admitted to probate unless the person propounding it produces evidence which is sufficient to remove the suspicion and to satisfy the Court that the testator both knew and approved the contents of the will.
4. The weight of the onus will be proportionate to the gravity of the suspicion raised in any particular case.
5. Neither the superficial appearance of lucidity nor the ability to answer simple questions in an apparently rational way are sufficient evidence of capacity.
6. The duty upon a solicitor taking instructions for a will is always a heavy one. When the client is weak and ill, and particularly when the solicitor knows that he is revoking an existing will, the responsibility will be particularly onerous.
7. A solicitor cannot discharge his duty by asking perfunctory questions, getting apparently rational answers, and then simply recording in legal form the words expressed by the client. He must first satisfy himself by a personal inquiry that true testamentary capacity exists, that the instructions are freely given, and that the effect of the will is understood.