Name changed for this as possibly outing.
So I've found WhatsApp messages on my husbands phone to a female friend.
We have been on the rocks for a while and no longer wear wedding rings.
Friend has known us 10 years and is 15 years younger than 'D'H.
Messages are everyday going back over a year. Many messages a day.
I know they are closer than other people in our friendship group ie interests and hobbies etc.
Messages are everyday subjects to more in-depth conversations.
No mention of I love yous or the like.
Good morning/night messages everyday.
Messages of just 'Xs'
Every message ended with Xxx
Some inappropriate messages about the woman's boobs and his name for them 
Other inappropriate and ambiguous statements.
Compliments from both sides.
Bitching about me and her OH.
One even said don't message yet she has my phone.
Arranging to meet for coffees and lunch.
Ridiculous nicknames for each other.
Very unlikely it's got physical.
Is this an affair?? Close friends? And should I call him out?
There are children on both sides so don't want to rock the boat.