I am seconding the book "Not Just Friends" by Dr Shirley Glass.
You two need to read it, together.
After reading it, you may find that their relationship is heading down the slippery slope.
Over sharing, confiding intimate marital details is an indicator. If the EA partner knows things about your spouse or your own marriage that you don’t know. If your spouse feels more understood by the other person.
Hiding, omission, or outright lying about extramarital relationship communications is another. Deleting comms, using covert comms, apps.
Poor communication boundaries such as late night comms, excessive comms, excessive terms of endearment, sexual innuendo.
Intrusive thoughts, preoccupation
If your spouse rigorously protects, defends or prioritizes the extramarital relationship over the marital relationship, over your concerns.
If the extramarital relationship exists at the expense of the marital relationship.
If the extramarital relationship seems to be evolving, growing, with parties progressively growing closer together rather than stabilizing, plateauing.
If comparisons are made by your spouse between you and the other party.
Who is the first and last person your spouse communicates with at the beginning and end of the day. How does the frequency of communications compare to marital comms?
As Tanner says above, this isn’t you first rodeo. Your senses are highly attuned to boundary pressures.
[This message edited by RealityBlows at 8:25 AM, Monday, December 18th]