You can't tell them it is time to split up! Your role is that of facilitator. You can help them to see that it's over. But that has to be their conclusion, their decision. And, trying to save a marriage should come first, unless one or both parties are not committed to that.
"Have you ever been separated?" "Have you ever given it any consideration?"
You have to be careful not to look like you are taking sides. Your schooling will have you do mock exercises where you will practice what to say and how to say it.