He must be mekubal.If he is someone like Rav Mordechai Eliahu his advice must be taken very serious. However, he was a rare exception (according to his aide Rav Zafrani there a only a few such people in each generation). In other words, he must be a gadol (in which case he will deny that he is a mekubal – and, in fact, Rav Eliahu did not discuss Kabbala with his son and heir Rav Shmuel, or at least Rav Shmuel says). Unfortunately, there are many charlatans who tell people breeak up marriages, rip off people for money and even tell people to do severe aveirot (with them and I will not go into detail).
BTW, neither Rav Kuk nor Rav Soloveichik would give absolute advice. They would set for the alternatives and their ramifications (which they received from their analytical abilities developed over years of learning at the highest levels) and then give the questioner responsibility for his life. Even Rav Eliahu gave alternatives. I had a neighbor who went to him on some personal matter and he suggested that she add a name. He then gave her a list from which to choose.