If the animal has been slaughtered properly, then it is permissible to use its horn. If the animal is still alive or it dies without being slaughtered in the prescribed manner, then there is a difference of scholarly opinion as to whether it is pure or not. For more, see the detailed answer.

Summary: meat slaughtered by one who is uncircumcised is lawful and may be eaten, whether he is a Muslim or a kitaabi (Christian or Jewish).