Weddingbee boards

Need your advice!!  I was out this evening catching up with an old girlfriend.  She has broke up with her ex & needed a friend. Her ex is good friend with my SO's lovely ex wife. 

She is the nice ex ( I think she's fake & I have always had an unnerving feeling about her) my SO is blind & says they are just friends. She was texting him almost daily & was always super nice to him & nice to me when he is around. 

He as slowly been ignoring some of her silly messages or letting her know he already knows.   Shortly after we got together he asked her for a divorce.  I think he loves me a lot!!!  Worried he is so friendly to her because he may still have some feelings for her.  So anyway... When I mention to my frend tonight how "friendly" my SO's ex is with him she tells me the his ex-w told her bf that she is having a hard time getting serious about her new man because she is still in love with my SO!!  I totally knew it!!   Do I tell him???  So he respects my requirement for appropriate space? Or do I keep it to myself  & let him keep think she just wants to be his good friend?