This is a more complicated question and the answer is extremely difficult. The answer to this question is wholly dependent upon the individual. Just because someone does not keep Torah does not necessarily mean they are not a believer. There are many different reasons that people do not observe the Torah. Only one would condemn a person: hostility toward the Torah itself.
Millions upon millions of misinformed and mistaught Christians live in ignorance. They believe with their heart unto righteousness, they love YahShua and they are truly doing all they know to conform to His law. The problem is, their knowledge of the law is extremely limited. YahShua will only judge us for what we know. He is a fair and just ruler. (To whom much is given, much is expected.) Many believers simply do not know they have an obligation to keep Torah. Of course scriptures in both the "Old" and "New" Testaments make it clear that we must keep Torah, but these scriptures can be muddied by wrong teaching.
Believers in the Messiah who simply do not know or are not yet convinced that Torah observance is necessary (due to lack of teaching or much bad teaching) are not guilty of sin. If they are faithful to pursue YahShua and read His Word and stay close to Him, there is a good chance that they will eventually discover that the Torah is eternal. As we near the end times, YahShua is making Torah observance more and more clear and common to the believing community. As the light brightens, more Christians will have the opportunity to see their error and change their ways.
A Christian who doesn't know better or has been loaded down with bad teaching (as most are in this culture), should not be treated as insincere. It took me a good year and a half before I was completely convinced I needed to keep Torah. This type of Christian is very much like a baby. Do we punish a baby the same way we would a teenager if they do something "wrong"? Of course not! The baby does not know better and must be patiently taught, instructed and trained. Only after they aquire the knowledge do we inflict consequences should they do wrong.
Our entire society is based upon this. We punish people more severely the more knowledge they have that they willingly transgressed. Ignorance may not be an excuse to break the law, but it certainly factors into the punishment received. This is just common sense.
Christians who are ignorant or just filled with bad teaching need lots of prayer and patience. Spend as much time as is needed with these people to demonstrate through love, Scripture and life that keeping Torah is part of the New Covenant, too. As time progresses and you shine the light of knowledge more brilliantly upon your Christian friend, he will either become more open to the idea or more hostile.
Even those who are initially hostile may just be trying to play the devil's advocate. Don't write them off just yet. Continue to pray for them, be patient, and show them over and over from Scripture why we need to keep Torah.
Once a Christian shows open and sustained hostility to Torah keeping, you can be more and more certain that they are not true believers at all. In these last days, I believe the Torah will bring great division within the Church. Two camps will form, one for the Torah and one against. The anti-Messiah will be a Jesus who rejects Torah. YahShua is one who embraces Torah. The Torah will polarize the body of believers and sift out those who love Him and those who don't.
Those who actively take a stand against the Torah are in a very dangerous position. You should warn these people strongly and pray for them even more. Undoubtedly, keeping Torah and instructing others to do the same will bring much persecution. Many Christians will not tolerate this teaching and will try to oppose you in any way they can. Do not be discouraged. Standing on the Torah is standing on the Rock. Once a person actively opposes the Torah, do not debate with them any longer or you will only serve to harden their heart further. Warn them once and then pray for them until they change.
Remember, it takes time to bring an ignorant Christian into even the initial understanding of what it means to keep Torah. As long as they are still willing to discuss the issue with you, keep on patiently sharpening them. Most Christians are very open to the idea and will be interested in what you have to say if you deliver it in the spirit of humility and love.