In order for the scenario to cause as little problem as possible, Bill should have put in place a Lasting Power of attorney, prior to losing mental capacity.
Many may think that making a Will would have made the scenario easier. However, if a Power of Attorney had not been a place, an application to the Court of Protection would still have been required if Janice had left her estate to Bill as someone would have still needed to deal with his financial affairs. However, that is not to say that making a Will is less important as in fact a Will does make the situation easier whether the beneficiary has mental capacity or not.
