If three men sat together to eat a meal with bread and two of them ate permitted Kosher food, but one of them ate forbidden food, may he combine with the other two in order to recite Zimmun before Birkath HaMazon (the invitation to say grace which requires at least three who have eaten bread).
The Shulhan 'Arukh writes that if someone ate something forbidden, he may not combine with the others to say Zimmun. This does not only apply in a case where the person in question ate something forbidden by the Torah, but it also applies if he ate something forbidden by Rabbinical law.
Not only that, but a blessing may not be recited over such food, before eating it or after eating it. If the person eating it does recite a blessing, one should not answer "Amen" to his blessing.
(See Shulhan 'Arukh 196:1. Mishnah Berurah, ibid., 1, 2)
Birkat Hamazon, non-kosher food,