a chunk of that says I should act asap to stamp on the EA.
This is true. If it's confirmed (and it sounds like it is) then yes. How you handle it is the important part. So gather the evidence and then make sure it's water tight. Note, a WW will spew and scream because 'They are just friends' and 'There is nothing going on' Etc. Be careful of how you play this because if she's ever accused you of being controlling or manipulating then this will get thrown back in your face. So be like Clint and make sure she can't deny it.
Me:43 Her:42 M:14 S:9 EA started 2014/03 (or there abouts) PA started 2014/05/30 BD:2014/11/05 I left 2015/10/01 I returned 2015/05/02 She left 2015/06/10 OM still on the go.