In my own defense ...she already knew and made it beyond clear by way of first stating she was looking at a picture of me attached to my phone number on his phone when she called.
What we don't know here is if she actually had a picture of you
or found your number on his cell phone. Could it have been, for
example, a number she found on a piece of paper in his pants
when she was doing laundry?
We also don't know what she thought prior to making the call to
you. Perhaps she was calling because she was curious or suspicious
and wanted to find out what her husband's relationship to this
phone number was all about?
Even if it was as she stated, a pic and number on his phone, she
could have still been ignorant as to the relationship involved.
For all she knew it was a business associate, old friend, distant
relative, his personal hairsytlist or massagist, etc. Of course if it
was a nude pic then he would have had a harder time explaining
it away. What could he say then? That it was from a porn site?
In conclusion, by agreeing to speak to her in the way you did, it
seems possible it may have been the clincher in a series of events
leading to his cheating being known to her. IOW it seems possible
that you may have outed him. Why else would she call you, except
to find out if he was cheating?
Although, as has been suggested earlier in this thread, it may be
that he purposely left clues for her because he wanted her to
discover his activities.
Certainly they put you in a difficult situation, and you acted in a
compassionate way. In the end, who knows, this may work out
to have been the best thing for that marriage. And the way you
spoke to her being one reason for that.