Is it possible that I have afflicted someone with the evil eye, causing them to suffer a heart attack, and how can I perform ablution for him while he is in this condition?

If it is merely a doubt, then nothing is required. However, Islamic Ruqyah is recommended, as Muslim narrated that Jibreel came to the Prophet (peace be upon him) and performed Ruqyah on him. Among the methods of treatment are: reciting Ruqyah over water, then the afflicted person drinks from it and bathes with it; or reciting Al-Fatiha, Ayatul-Kursi, the concluding verses of Surat Al-Baqarah from Allah's saying: "Amana Al-Rasool" to the end of the surah, and His saying: "And indeed, those who disbelieved would almost make you slip with their eyes," along with Al-Ikhlas and Al-Mu'awwidhatain, and some prophetic supplications. It is also possible to wash some limbs in a vessel, then fill a bottle with water from it, then recite Ruqyah over it and blow into it. Then the afflicted person bathes with that water, hoping that what ails them will depart, by the permission of Allah Almighty.