Usually the claims regarding the Law not being abolished is about meats and days, and maybe sometimes physical circumcision.
As an ex-Sabbathkeeper, I'd recommend these resources if someone is seriously troubled by this issue:
Sabbath in Christ, Dale Ratzlaff I don't agree with every point, but in general it is a very good introduction to the topic by an ex SDA
The Sabbath Complete: And the Ascendency of First-Day Worship, Terrence O'Hare
A sermon series entitled "The Fourth Commandment" by Dan Caffese on SermonAudio
Dan Caffese Sermons | SermonAudio.com
A summary of my position is that the Sabbath has been fulfilled in Christ, who is our spiritual rest, and that God is not concerned with days and meats. Additionally, the Mosaic Covenant is not in effect anymore, and those elements are part of the Mosaic Covenant.
You can debate alleged Torah observers for pages and pages on this topic, though. I was as bull-headed as an Armstrongite, and thought there was NO POSSIBILITY I was wrong on the topics of Sabbath, festival, and clean meat laws observance.
Every single argument they present can be refuted.
If, however, they want to observe their pick-and-choose, cafeteria plan Mosaic Covenant without making accusations about non-observers being in sin or unsaved, I have no issue with them. There may even be value in observing the shadows and types of the Mosaic Covenant. For me, though, I think it logically leads to looking down on non-observers as inferior or even unsaved. And since I abhor that aspect of it, I won't have anything to do with it.