You could easily have been with someone since you were 15, be 50 now with primary aged children.
Trust your gut, but not a word.
Better to do as advised, confide that you were hoping to get back together but are so upset to hear that he was seen with someone that DOESN'T have her hair colour, 10 years younger, in a village nearby.
Be gutted, but say I knew there was someone else, and make up the identity of the person who told you.
Also tell her he was far more interested in sex before he left, which can be a sign of an affair.
Be very disappointed, but watch her face very carefully.
Preferably someone 10 years younger.
How upsetting, but I would trust your gut.