My husband is sexting another woman

My husband has been sexting. This woman is in a loveless marriage and had been seeking support from my husband. I did know that she sometimes spoke to him and she has been a family friend for a long time, but I didn't realise they had started sexting.

My husband says he's been a fool and he's really sorry. He loves me and him wishes he hadn't done it and is devastated he has hurt me. But I can't get my head round why they were sexting? He must fancy her or is it just a dirty text that he got turned on by and was flattered by?