Hey there,
Maybe just ask them why they believe that, and ask them to show you in scripture. It'll probably be Hebrews 4, then be prepared with a deeper understanding of that scripture than they are gleaning. Then go from there with why it is you do keep it. If you ask how they apply the 10 COmmandments to their lives, it will give you a good indication of where to start. If they believe the 10 Commandments apply to us NT Christians, then you can skip over geting to that point. It might also be a good idea to ask that person if they were to discover that the Sabbath was binding as it is written in the decalogue, if they would make the change and keep it. It could show you whether or not they are even open to following the truth or not.
I am currently attending a Christian home group with Sunday keepers, I just started. Something in me thought it would be a good idea. And lo and behold, they are doing a study on the 10 Commandments. I came in on Thou shall not murder. I have this week, then the following one will be on the Sabbath command. Should be an interesting discussion. I'll be in the same boat as you.
I think the key thing to remember is that God loves everyone, and would have everyone keeping the commandments. It's very easy to get frustrated and take an adversarial attitude, or to get exhasperated (if you are anything like me) - there's just so much in the Holy Scriptures to get to regarding the Sabbath, and it can be overwhelming to have so much information and help to give someone in one sitting. I'm trying to keep it in my heart that if it comes to me to shed some light on the Sabbath, then I need to love the people I'm talking to, and respect that they are being called just the same as me. Give it according to the Spirit so thay can hear it according to the Spirit. If I'm debating rather than showing, I'm giving it according to the flesh, and they will most likely hear according to the flesh - not good.
And trust the Holy Spirit to bring up the knowledge we already have, in the way and order that will be best for the people we're addressing. Flying by the seat of the pants!
I don't know how useful it would be to just throw out some scripture that contradicts the way they have interpreted the Sabbath and the NT. They have a Bible and they probably already read it. They are probably equipped with their responses and a tit for tat, might just be a dead end. Don't know though. There are whole books written on this topic. Maybe presenting this person with a good book. I guess it depends upon the person.
I actually had this conversation regarding the Sabbath with two Sunday keeping pastors. They are educated in scripture, though lacking in knowledge, and they are rock solid in their defense of their views and decisions. People who rely on their own understandings and the traditions of men at the expense of following the commandments of God might be shutting out the Holy Spirit and be unaware. The best solution I can think of is to honor what they think they're doing, which would be loving God, and pray, and just know your stuff so the Holy Spirit can use the knowledge you've been given, to help someone else.

? Did that help at all?

Donna