If he's like any of the 20year olds I know he could have been having a dozen whatsapp/text chats, many of them group chats with 5-10people one for friends, one for colleagues, one for family, then subsets of each of those to cut out the people you really want to chat about, have you tried working with someone whose phone pings that often - nightmare!
As it was a picture message not a whatsapp they will know who it was sent to from his phone records if not from the recipient themselves so for me that rules out a hookup that then goes bad as they would know exactly who it was.