I understand from your question, including the other information you provided in your question that I have omitted here, that your own food is kosher and that up until now you’ve been making blessings on your food in the presence of your friends. They find this behavior praiseworthy and would like it to “rub off” on them in a positive way. I commend you for making your own blessings and not being shy or embarrassed to say blessings in public in their presence.
What should you do about their request to say a blessing for their food as well? Here’s what I suggest:
Only a Jew has a mitzvah to say blessings before and after eating. A non-Jew does not have this obligation according to Judaism. However, I am not aware of any prohibition for a non-Jew to thank God for his sustenance by saying a blessing. A non-Jew has no restrictions regarding non-kosher foods.
Having said that, it is not correct for a Jew (i.e., you) to make a blessing over non-kosher food if that is what he eats. Why? Since God told the Jewish People to observe the laws of eating only kosher food, it is not desired by Him when a Jew chooses to eat something not kosher. God doesn’t want thanks or appreciation for a Jew when eating non-kosher food since He’d rather the person not eat it. In a sense, making a blessing to God for non-kosher food would be akin to spitting in someone’s face.
Therefore, I suggest that you tell your friends that you are honored by their request and are happy to say the blessing for the food you are eating. You should say the blessing for your own food, in Hebrew as you usually do, and they can say “Amen” when they hear you complete the blessing. Your blessing is for your own kosher food, which is correct, although it is really unnecessary for the others since they have no mitzvah to say a blessing. But if they want to hear your blessing that you are anyway saying for your food, and answer “Amen”, that is fine. Although they have no obligation, it sounds like a good intention to be included in your act of thanking God for the food He has graciously provided for all.
An aside: I don’t think you should “use” your blessing to exempt you from sharing in any financial tip you all leave for the food server. A mitzvah is not a “tool” to be used to benefit from others. But I hope you find my “tip” clear and helpful, although I wouldn’t expect your blessing to improve your golf score!
