Yes, atheist can know right from wrong because the Bible says so.
Romans 1:14-15, "For when Gentiles who do not have the Law do instinctively the things of the Law, these, not having the Law, are a law to themselves, 15 in that they show the work of the Law written in their hearts, their conscience bearing witness and their thoughts alternately accusing or else defending them."
The Bible tells us that everyone has the law written on their heart. The law is those do's and don'ts of moral behavior. Consider the 10 Commandments in Exodus 20:1-17 which tells us such things as do not lie, steal, or commit adultery. Though not all unbelievers acknowledge all of the moral truths in those commandments, they sufficiently recognize moral truths and show they are written on their own hearts.
However, they suppress the knowledge of God and moral laws of God in there unrighteousness.
Romans 1:20-21, "For since the creation of the world His invisible attributes, His eternal power and divine nature, have been clearly seen, being understood through what has been made, so that they are without excuse. 21 For even though they knew God, they did not honor Him as God or give thanks, but they became futile in their speculations, and their foolish heart was darkened."
There might be atheists who assert certain moral values such as saying it is wrong to harm someone against their wishes. But this commits the fallacy of begging the question. This fallacy is the error of assuming something to be true that you're trying to prove. In this case, atheists will often say that the goal they seek is the right one, the goal of not harming people. But What makes that goal the right one? They just assume it's the right one and argue that point.
Though we admit that people generally don't want to be harmed needlessly, simply because someone wants or does not want something has no bearing on whether or not it is morally right or wrong. This is the problem with the atheist attempted to justify what he believes is right or wrong.
When man is the standard of righteousness, then he must be the terminus. This means that there can be no no means to establish moral truth beyond his own existence. This also is a form of begging the question and is fallacious.
So, can an atheist know right from wrong? Yes she can. But the reason is because the law of God is written on her heart, not because she can justify what is right and wrong.