Answer

In the name of Allah the Provider

If the person is eating deliberately then he should be told that it is Ramadhan. To eat in Ramadhan knowingly is a grave sin.

However, if the person is consuming food out of forgetfulness then the ruling is that if the person is weak (meaning that he appears weak and frail) then the onlooker should not inform him, because it will be deemed that Allah fed that person.

If the person who is ingesting the food is not weak and frail and he has the capability to complete the fast, then he should be reminded that it is Ramadhan and that he ate out of forgetfulness. But in England and many developed countries in the world today, one will seldom find someone who is considered weak and frail even if they appear thin, they will be in good health and competent to conclude the fast. Hence such persons should be informed that they are eating in Ramadhan. If they stop then their fast is valid, if they continue to eat and consume food then their Fast is broken.

And Allah knows best

Shami, Kitabus Sawm, Vol.2
Noor-ul-idha pg 141

30th Oct 2005