Words are very important here. Because of very real differences in faith, it is not possible for Christians to pray with those of another religious tradition. However, we may, and in fact are encouraged to respectfully be in the presence of people of other religious traditions when they are at prayer, just as we would encourage them to observe us when we pray. A very helpful book in this regard is How to be a Perfect Stranger: A Guide to Etiquette in Other People’s Religious Ceremonies (Arthur J. Magida, ed., Jewish Lights Publishing, 1996, 3rd ed., 2003).
The same principle applies with saying grace. You may certainly respectfully observe while someone from another tradition blesses the meal. Likewise, if they come to your house, they will no doubt expect you to say the traditional Catholic Blessing Before Meals as well the Thanksgiving at the End of a Meal.