All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
First of all, you did not mention the statement quoted from Imam Ahmad so that we would clarify to you whether or not it is true and clarify its meaning to you.
In any case, it is permissible to eat the meat of animals slaughtered by a person from the People of the Book, meaning a Jew or a Christian.
Therefore, it is permissible for a Muslim to help a person from the People of the Book in slaughtering the animal and eat from it afterwards, whether he did the slaughtering himself, or it was done by the Jew or the Christian.
As regards a disbeliever who is not from the People of the Book, it is not permissible to eat the meat of an animal slaughtered by him. However, if the Muslim is the one who slaughtered the animal, then it is permissible to eat from its meat.
There is harm if the disbeliever helps the Muslim in other than the slaughtering, such as holding the animal and so on. However, if it is the disbeliever who slaughters it, then it is not lawful and it is not permissible for a Muslim to eat from it.
Allah knows best.