I'm very sorry this has happened op, especially just before Christmas.
(Christmas always seems to bring the out the lovey dovey idiots).
It is completely up to you how you proceed, we have all been there, that awful knot in the stomach, the feeling that you just want things back as they were, its truly heart breaking.
The posters are not trying to be hurtful they are just warning you as there are so many alarm bells ringing. We don't think his lesson has been learnt and believe any further action of his will simply go underground.
This is the part where so much damage can occur to you, the gaslighting and lies will floor you even though you think you are in control of the narrative. Many of us thought that, he wouldn't, couldn't throw everthing away even 30/40 years together, children, grandchildren, we are different. They do.
You are in a possition before you have children to decide whether this is the partner you want for life, and this is probably why posters are nervous for you, it feels like there is a time limit and we know the lies could go on for years, he is not the partner you deserve.
You are questioning your ability to get someone else but I want to tell you, you are better than him, his morals and strength of character are flawed, he doesn't deserve you.
I really hope he can turn things round and this is just a blip but I would want to know everthing before you put it to bed and that would require speaking to her partner and knowing whether she has also lied.
I fear she has lied and that means they will both lie again.
Whatever you decide is your choice no one will judge you, and hope you feel you can return for support on this thread.