It's Lent. A non-Catholic friend of yours has invited you and your beloved to dinner at his house on Friday night. When you get there, you find that the entrée is steak. Your friend grills an excellent steak. But it's a Friday in Lent. What do you do?
Eat the steak. Enjoy the steak. Thank your host for a lovely meal. Don't bring up "meatless Fridays" unless you can segue to it naturally and charitably from some topic being discussed; better to wait for another day.
Nowadays, with the greater social concern over food allergies and dietary restrictions, it's becoming more common for hosts to discuss preliminary menus with their guests unless they know each other so well that such things are already known. So this situation is becoming less common.
The practice of meatless Fridays is no longer enforced under penalty of sin outside of Lent. However, it's still a very, very good spiritual practice, a minor mortification offered in union with our Lord's suffering on the Cross (cf. Col 1:24). (For a Nebraskan, giving up steak at any time is almost a heroic sacrifice.) It's also one of those culturally distinctive practices that promote Catholic identity.
Having said that, though, even if it were still being enforced under penalty of sin, I would recommend this course. The overriding principle here is charity.
Your host was under no obligation to invite you to dinner; once you committed to the dinner, you committed to the menu. If you'd had reason to suspect that your friend would use the dinner as an opportunity to test your faith, the appropriate response would have been either to suggest Saturday or beg off on the grounds that you would not want to put him out of his Friday steak. As it is, the charitable assumption is that he either didn't know or forgot that you were keeping the practice.
As for the man who is weak in faith, welcome him, but not for disputes over opinions. One believes he may eat anything, while the weak man eats only vegetables. Let not him who eats despise him who abstains, and let not him who abstains pass judgment on him who eats; for God has welcomed him. ...
Then let us no more pass judgment on one another, but rather decide never to put a stumbling block or hindrance in the way of a brother. I know and am persuaded in the Lord Jesus that nothing is unclean in itself; but it is unclean for any one who thinks it unclean. If your brother is being injured by what you eat, then you are no longer walking in love. ... Let us then pursue what makes for peace and for mutual upbuilding. Do not, for the sake of food, destroy the work of God. Everything is indeed clean, but it is wrong for any one to make others fall by what he eats; it is not right to eat meat or drink wine or do anything that makes your brother stumble. —Romans 14:1-3, 13-15, 19-21