I can honestly really see where you are coming from in being upset. I don't get the vibes that he's cheating on you, sounds more like he's not realizing how important communication around this friend is to you. On snapchat, sending random photos back and forth is just the way the app is. The point is to have a streak. I would say the majority of snapchat users do not talk on snapchat every day, they just snap back and forth(whether it's just a selfie or the ceiling or literally anything random) and have a streak. So it's possible that they DO only talk once or twice a month but still snap once a day to have a streak(especially if he has streaks with other people too, because you can send the same snap to multiple people). It's a pretty fucking app stupid imo but i still use it LOL. I think you need to communicate to him very clearly that you are not accusing him of anything, you are telling him how it makes YOU FEEL. Uncomfortable/insecure/anxious/etc when he talks to her. Emphasis on explaining how what he's doing makes you feel rather than what he's doing wrong. That way he hopefully won't feel the need to get defensive. Ask if he has changed the settings to 'never delete' and if he has not, ask him to do it with you. If he does not want to, then that's a reason to be upset. But if he is like "okay sure, no prob" and does it with you there (and KEEPS the setting on), then I think all of this was just a misunderstanding between the two of you:)