ANSWER:
This is not sukoon rather Satan is making haram feel like it to make him fall into more and more haram until there’s point of no return for him. It is an addiction to filth same as the addiction to alcohol or drugs, as those who do such intoxicants also claim to find sukoon!
This kind of relationship is absolutely prohibited in islam and he is sinful for continuing this. If he does not want Allah’s wrath on him in this dunya and the hereafter and if he wants Allah to pardon him, he must immediately stop all communication with her immediately and block her from everywhere besides repenting sincerely.