Christ gave up his life for that person. Wouldn’t you at least be willing to give up going to dinner for him—because, as you say, it doesn’t really make any difference? But it does make a difference if you hurt your friend terribly, risking his eternal ruin! When you hurt your friend, you hurt Christ. A free meal here and there isn’t worth it at the cost of even one of these “weak ones.” So, never go to these idol-tainted meals if there’s any chance it will trip up one of your brothers or sisters.