Praise be to Allah, and peace and blessings be upon the Messenger of Allah and his family.
Firstly:
In order for it to be permissible to eat chicken and meat that is sold in Muslim countries and elsewhere, two conditions must be met:
1.The meat should have been slaughtered in the manner prescribed in sharee‘ah; it should not have been stunned with electricity, or drowned in water, or any other means of killing that is done in some countries.
2.The one who slaughtered the meat should have been a Muslim or a kitaabi (one of the people of the Book, i.e., a Jew or Christian); it is not permissible to eat meat slaughtered by a communist, atheist or idol-worshipper.
See the answer to question no. 83362 .
If it becomes clear to you that the chicken sold in these restaurants has not been slaughtered in the manner prescribed in sharee‘ah, it is not permissible for you to eat it and it is not permissible for you to work with those who sell it, because that comes under the heading of cooperating in sin and transgression.
Secondly:
It is permissible to eat fried meat and fish from these restaurants, subject to two conditions:
1.That it is not fried in the same oil as the non-halal chicken or in vessels in which chicken was fried, until they have been washed, because the non-halal chicken is maytah (dead meat) and is najis (impure).
2.The meat should be halal.
And Allah knows best.
