The 4th does say "On it you shall not do any work, you, or your son, or your daughter, your male servant, or your female servant, or your livestock, or the sojourner who is within your gates"..... so if someone is serving in any manner not related to acts of mercy or need etc then they are acting as servants who should rest.
So would you agree with my analogy? I would take your position Sarah, that the server in a restaurant is sinning by working the unnecessary job on the Lord's Day. But the problem for people I talk to is that even when they buy that position, they don't see their eating there as being sinful since they aren't the one working.
I would agree with your analogy in that they approve of the sins of others and give rise to that sin by eating at the restaurant which Romans 1:32 (Though they know God's righteous decree that those who practice such things deserve to die, they not only do them but
give approval to those who practice them ") condemns. I guess I don't understand their position entirely. Either they believe in the 4th or they don't. They can't have it both ways by cutting it in half and leaving part of it intact for themselves and ridding themselves of the other half when it comes to other ppl. Even when I didn't believe in the Sabbath I never cut the law in half saying I can follow the 4th but I'm not going let others do so. I have to say that would be very selfish indeed to allow rest from their own work to enjoy the Sabbath but not allow rest to others for the studying of God's word or other writings and fellowship with the brethren not to mention they have just sliced and diced God's law and approve of such things in others.
If they don't believe in the 4th, well then you have a whole set of problems on your hands. It took me a long time to believe the 4th was necessary and I still am not the hardcore 4th keeper as maybe some others are. What convinced me of keeping the 4th was John Calvin. As others thought so did I in the past on his softness of the 4th. But he wasn't I just didn't understand him rightly at the time, bc I was so focused on proving the 4th was no longer valid physically and it was a spiritual rest from our works. What I did see in his writings at the time that did convince and maybe will help your friends is this: (remember this is what I thought back a few years ago)... "he didn't really care if ppl wanted to make Sundays the Sabbath bc taking at least one day to do nothing but spend time in church and dedicating oneself to the things of God was the most profitable thing to do for oneself and to not keep others from doing such things." And so I slowly came around to keeping the 4th on such an idea. What a wonderful concept it would be to devote one day completely to God and to allow others the same benefit. It rang true to my heart even though my mind continued to reject that it was called the Sabbath. After sometime I eventually just accepted the Sabbath as being such and not too long ago my pastor gave the best reason why the Sabbath day was changed from Sat to Sun. It use to be Sun because God rested from his work after working six days and commanded that we mimic his actions. Then it changed to Sun bc Christ died and rose again bringing in a new era where we rest from our work right out of the horse gait without ever starting to work for salvation and rest in his work (of course, my pastor said it much better than i just have)...BAM! It connected with me finally bc it was the change of days that kept tripping me up. OK I've gotten off topic. ahem...
I would ask them this: "What about the Sabbath do you (that is your friend) believe or not believe or how would you define the Sabbath?" Allow them to state their position so that they can see from their own words how flawed their view on the Sabbath really is when you do explain it to them. Then I would ask them why they want to go out to eat on the Sabbath. Is it to have fellowship with the brethren? If so, what a better way of showing love through action than to prepare a home cooked for those same brethren in an atmosphere of quietness where they can enjoy each in Christ? If it is more a cultural thing as it is in the south, then maybe show them God's law prevails over culture. If it is just a treat, encourage them to treat themselves on another day. They may have an entirely different reason for doing it which you'll have to address. However, it all comes down to "are you making the Sabbath God-centered or self-centered?"