She's daft. Very attractive, married and has been for a long time - not unhappy but not overly happy either. Her life is run on her husband's say so - not necessarily controlling but things are kind of set up to suit him.
She has become very close to a (10yrs younger, married) work colleague; he emails her occasionally and pops in to her office (she works alone in the office) daily for a chat. He makes all the running but she doesn't knock him back.
There's a strong mutual attraction and she says if neither were married and she were younger, she'd absolutely go for it.
So, she offered him her number because she didn't want anything showing on her work email.
He says he can't/won't contact her via his personal phone. He's happy to have contact but only on his work email, she's happy to have contact but NOT on her work email...
He just doesn't want his wife to possibly have the opportunity to find out what he's doing, does he? 
I don't have an opinion either way re an emotional affair, that's up to her but I wonder if he's a bit of a player and hope it doesn't all end in tears.