Cindy seems to be having a mid-life crisis, despite her being only 25 or whatever she is. Unhappy with her job, disappointed by her live-in boyfriend, and looking for something in her life that can offer her excitement and an outlet for her creative "passion," she would probably be tempted to cheat on Wade with anyone who provided those things. Ian and Axel happen to be well-placed to deliver some thrills and their artistic/creative activities appeal to her. I don't think any of that makes Cindy a "shitty" person, or at least no shittier than Wade or Ian or Axel. Wade clearly has zero interest in sustaining a successful adult life and partnership with Cindy--witness his year-long jobless stint in which his only ambition is to compete in unpaid gaming tournaments, and he begrudgingly joins Cindy in nights out.
Ian's affair with Cindy is particularly sad because if discovered, it will not only destroy Wade and Cindy's relationship, it threatens to torpedo the whole friend group. But Wade and Cindy are sabotaging their own relationship, which appears doomed regardless of what Ian does. Given how young Wade and Cindy are and the fact that they've only been together a few years, the fact that they are struggling so much signifies that they are a bad match and that a breakup would be best for both of them.
I completely agree with this. I wouldn't call it a ''mid-life'' crisis per se, more of an identity crisis. I get the feeling she rushed into the life she wanted, or what she thought was expected of her (steady job, boyfriend, appartment), but is starting to realize she never really asked herself what she truly wanted. The same thing basically happened to me when I finished college and all those things I wanted to do ''when I will be older'', well I was in it and had no clue how to go about it. I certainly empatize with her helplessness.
As for Cindy and Wade's relationship, there's nothing to destroy. It's already dead, they just don't realize it. They've drifted apart, and Wade has shown no indication that he wants to save the relationship or even work for it. He doesn't care about Cindy, and only focuses on his needs. That imaturity has doomed their relationship. I completly get that Cindy feels caged in the sense that she wants to explore, do things, but she's being held back. Their break-up will probably be a reality check for Wade and hopefully, will push him to grow up. Though I agree, lines will be drawn in the group of friends if Ian's affair with Cindy is discovered. But in the long run, it will be better for everyone.
Would Cindy cheat with Wade with anyone? I don't know, but it's probable. She's in a fragile state and will embrace anyone that shows her how her life could be different. Manipulative assholes like Axel are also a danger for her, in her current state.