So to summarise if this is what you mean -
-You got together when you were 37 and he was 41.
-You knew then that he'd already 'broken up 3 marriages'- assume this means he had affairs as a single man with married women (but didn't end up with any of them.)
-OR it could mean he has been married 3 times and you were Partner No 4 but didn't marry.
-You had a child when you were 40.
-You carried on working to buy the house you share as tenants in common.
-He retired in his early 40s. He lived off the money he'd made in the oil industry (no clear how much) and he bought a house in Spain (unknown to you.)
-You decided to move out 6 months ago when he 'crossed a line' (unclear what.) This was a 'kill or cure' step on your part.
-But you've now discovered what appears to be an affair that he started 15 years ago.
Your question is- should you 'alert' this other woman to his past history.
Well, no, because she's known him for 15 years.
Why have you never married? Was he married 3 times or just had affairs with married women?