...and you saw something in their lives that was sinful, should you point it out?
Firstly, about time we Christians got on with making disciples of non-Christians. Otherwise known as evangelism. Great idea.
But, secondly, no. The problem a non-Christian has is that they're not a Christian. Their life is fundamentally opposed to God and any other sin in life is merely the fruit of that bigger sin. The heart needs to be changed before anything else can happen.
This is fairly widely understood - note the football pundit's disbelief as Joey Barton again claims to have turned over a new leaf, to have changed his ways. Maybe he can reform a bit, but he - like all of us - needs a new heart.
If we start targetting the apparent sins of people who aren't Christians the best we do is to replace 'sin' with the sin of self-righteousness. That's no progress at all. This was the tragic approach on Channel 4's Make me a Christian. More like, Make me a whitewashed tomb.
Show people Jesus, let him change their hearts and convict them of sin. And then help them along the way as the Holy Spirit transforms them gradually to be more human, more like Jesus.