OK
You don;t 'know' the situation with his wife unless you live in their house and share their bedroom - you know what he has said to you and the public impression he conveys about things.
He is married yet he actively looked for another relationship - if he doesn;t wan to be with his wife, then he should tell her and leave her, then he is free to pursue other relationships - he is doing it the wrong way round - no decent person does this.
He's keeping his options open - of things don't work out with the new woman(and they probably won't -it's not a real relationship, it secretive and illicit and therefore falsely exciting, that will change if he is free) he keeping his wife in reserve - that is shabby and disgusting way to treat another person.
He isn't giving his wife a choice. She may well not want to be with a man who shags random women he meets on the internet, she might find the very thought of it repulsive, but he's not giving her the option of leaving because he's lying to her. I presume that although things aren't working with his wife he still sleeps with her, (although he probably tells other woman and any one who will listen that he isn't) - that's having sex with someone without them having all the information they need to make a clear and informed consent. If she knew he shagged internet randomers, his wife might well not consent to sex with him. I find deceiving people into sex an utterly disgusting and shameful thing to do to someone.
Your friend sound like a total arsehole. His life will probably go totally tits up and he will have serious regrets about his actions - these thing sr arely turn out well. And his kids will suffer.
If he and his wife split because they talked like adults and jointly agreed to part, then they could be friends, the kids would be upset but would adjust.The way he is doing it will lead to terrible heartache for all concerned.Except the 'other' woman who will find herself a replacement ten a penny shagger online and get on with her life.