A gift in the will of a rich relative can change someone’s life. But it is not unusual for a person to change a will shortly before he or she dies, leaving friends and relatives feeling cheated. If this happens to you, there may be something that you can do about it — but it will not be easy.

Last August, Westminster Coroner’s Court heard how Patricia May, an anorexic patient, left £1.5 million to Peter Rowan, her psychiatrist. The court was told that Dr Rowan had been prescribing Miss May large doses of antidepressants and sleeping pills without informing her GP. The coroner recorded an open verdict and pointed out: “It is not the duty of this court to rule upon any ethical considerations raised in the course of evidence, still less the validity of the will. Any proceedings, if they occur at all, may follow this inquest.” An investigation by the General Medical Council into Dr Rowan’s conduct is continuing. It is not yet known whether Miss May’s family will dispute the will.

Stories such as this are not uncommon. Well-known examples include Lesley Burke, an air hostess who claimed that she had been married to a Saudi millionaire for 15 years before his death in 2001 and produced a will in which her former “husband” had left her his millions. After two years of litigation she withdrew her claim.

In America, the dispute over Marlon Brando’s estate continues amid allegations of fraud. Loneliness or ill-health can make the elderly vulnerable to opportunists. According to documents filed at Los Angeles Superior Court, Brando was reportedly “incapacitated, confused, medicated and non-communicative” when he signed the codicil amending his will.

It is normally impossible to find out what caused the deceased to execute his or her last will. So what should you do if you suspect that a friend or relative is being persuaded by illegitimate means to make a will in favour of someone inappropriate? He or she may not take kindly to being told that the motives of a trusted friend or carer are dubious. If you believe that he or she lacks mental capacity to make a will, it may be possible to ask a solicitor to arrange for the Court of Protection to draft a will for the person.

If concerns arise after the person’s death, there are several grounds for having the will set aside. You may be able, for example, to show that “undue influence” was exercised over the deceased. But it is perfectly permissible for someone to encourage a friend or relative to write a will in his or her favour by reminding them of family ties, future needs or services rendered. For “influence” to be “undue” it must amount to coercion, preventing the author of the will from making his or her own decisions. That will be hard to prove.

It may be easier to challenge a will on the basis that the deceased did not know and understand its contents. This ground is often used if there are suspicious circumstances; for example, where a person who has been instrumental in drafting the will receives a substantial benefit from it. In the 2002 case of Fuller v Strum, a son challenged his father’s will on the ground that it contained a comment that the father hated the son “like poison”. The judge found that the father would never have made such a comment and set aside part of the will. However, the decision was overturned on appeal.

Another ground for challenge is that the deceased lacked mental capacity to make a will. This is increasingly common as people are living longer but not necessarily retaining their mental faculties. The test for ascertaining whether a person is mentally able to make a will is based on a case in 1870, Banks v Goodfellow. Broadly, a person must know that he is making a will, be aware of the extent of his property and have an understanding of whom he ought to be leaving his money to. In some circumstances, however, an elderly person may feel abandoned by relatives and former friends and may understandably want to leave his or her estate to a carer or adviser. If this happens, it is not uncommon for thwarted would-be beneficiaries to react badly. People often believe that they are entitled to be remembered in their relatives’ wills and may level accusations of impropriety at people they believe to have inherited what was rightfully theirs.

What should professional advisers and carers do to avoid being accused of impropriety? First, follow the rules laid down by your profession’s governing body. Failure to do so is likely to lead to disciplinary action. Doctors, for example, should not encourage patients to give them anything and should not accept any gifts that may affect, or be seen to affect, their judgment. They must also declare any gifts that they do receive.

Clients or patients should also be advised to obtain independent legal advice. Do not help the client or patient to give instructions to solicitors drafting the will; and in fact you should not even be present when those instructions are given. In recent years the courts have laid down a number of rules that solicitors should follow when drafting wills for elderly or ill clients, including obtaining medical confirmation that the person is sound of mind.

Wills may continue to be contentious. But people do not really want to use up what they should have received — in advance on lawyers.

Charles Lloyd is a partner and Jonathan Pratt a solicitor at the City law firm Macfarlanes

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