I can relate. I cheated on my now ex girlfriend, lied to her about it for some time and she also saw a video that i had taken of the act later on! I was completely awful to her. I was 100% confused about what I wanted out of our relationship, i tossed her feelings aside and was extremely selfish. I had cold feet and I made the biggest mistake I've made in my entire life. I will regret my actions till the day I die. We tried to make amends and heal thru the pain but she wasn't ready for therapy together yet and after she kicked me out of her place I had to stay with my mom for about a month till I found a new place. She expected me to drop everything anytime she demanded and basically used any small thing that disappointed her to unleash the massive amounts of anger she had for me because of this situation. She would say hateful things constantly (which she never did before i told her the truth), she would get jabs in every chance she could, she'd tell me she hates me, she'd tell me I am a piece of shit, she'd tell me she wants to kill herself and I'd just tell her I was sorry and that I loved her thru all of this, but the day before she'd say she wants to try and make things work! We did this for about another month. I kept my anger in check very well, she was absolutely trying to provoke me constantly and justified her behaviors by saying "this is the one time I am allowed to do this to you!"... there are some things I wish I'd have said better or differently but overall I'm proud of how self-aware I was thru the entire process and tells me I'm making good progress with therapy. In fact my doctor after 2 months wants to half my sessions already! There was nothing I could do right after the fact, she expected me to be perfect and while I was trying she was too angry to want to move forward and interact with each other in a manner that was conducive to moving forward. I hurt her more than anyone else in her entire life thru what I did. I'm in therapy now and I believe she still is too. Therapy has been very helpful and I imagine it would be for this guy too.
Anyways, the reason for the backstory is that I can relate to his guilt. I know that what I do has nothing to do with her at this point just like he does. That being said i went out a few weekends ago and there were some attractive single girls where I went. I figured I'd distract myself to get some positivity in my life (its been pretty negative and depressed since i told her the entire truth). I couldn't even do it. I couldn't flirt at all. It just wasn't in me. I felt awful. I felt like it's not fair of me to flirt with other girls and have fun since she's probably at home being miserable. I felt like I shouldn't be even SMILING or happy because of how terrible she is feeling.
That being said. when I talk to my therapist about these feelings he tells me they are completely normal and that I need to do what I am able to do to stay positive and not worry about how she's feeling so much. Worrying about how the old partner is feeling does NOTHING for the old partner. It's not doing a "service" to them to feel awful based on the fact that THEY are feeling terrible. You can still love your old partner and have a special place in your heart for them, even while moving on and being happy. Hopefully this helps. Good luck to you and your new friend.