Answered by: Mufti Mohammed Tosir Miah
I want to ask if we see someone else who is fasting eat or drink accidentally should we remind that person that he is fasting or not.
In the name of Allah, the Most Gracious, the Most Merciful
One should remind a fasting person if they are seen eating or drinking by mistake.
Breaking one’s fast ‘accidentally’ requires qadha but does not necessitate kaffarah. Accidentally means one is conscious of the fact he is fasting but broke the fast without the intention of purposely doing so.
For example, a fasting person is making wudhu and swallows water by mistake when gargling although he is conscious of his fast.
Forgetfully breaking the fast does not require qadha or kaffarah. Forgetfully means one is not conscious of the fact they are fasting. 
Only Allah Ta’ala knows best
Written by Mufti Mohammed Tosir Miah
Darul Ifta Birmingham
“مالوأكل” الصائم “أوشربأوجامع” أوجمعبينها “ناسيا” لعمومهلقولهصلىاللهعليهوسلم: “إذاأكلالصائمناسيافإنماهورزقساقهاللهإليه” فلاقضاءعليه
حاشيةالطحطاويعلىمراقيالفلاح،ج ١/ص ٦٥٨،المكتبةالشاملة