Empathy means being able to look at things through someone else’s eyes and trying to understand it from their stance.
We may not agree with them but be able to understand how they feel, because we have all experienced feelings such as sadness, hurt, envy, anxiety, fear, etc, and can understand how bad a person experiencing any combination of these feels. For instance, imagine your friend is in touch with someone you do not approve of. You have told your friend that this is not a suitable person time and again and that they might hurt them. Now, you are seeing that your friend is very sad and upset. You realize that the person has left your friend and robbed him too. Under these circumstances, if you tell your friend “I told you so”, it does not show empathy and it is also not a useful thing to say, because it does not help the person or situation. You can feel how much pain your friend is in and how much the incident has dispirited them.
So, if just stay by their side, listen to them talk, let them cry on your shoulder, and tell them you know how they feel without blaming or judging them, you have been very empathetic.