Ehh there's literally just two points where Ian needs to be a positive influence:
* "You need to fight for Cindy" in Chapter 4, when talking with Wade
* "You should bring Wade" or "You should ask Wade instead" in Chapter 4, when talking with Cindy
this sets Cindy/Wade relationship in max possible "they're both working to patch it up" state.
If you somehow mess it up and let it (partially) deteriorate through Ian acting dumb, you get a chance to fix it:
* "Do you really want to be with Wade?" in Chapter 9, when talking with Cindy
It's hardly any effort. Probably harder to keep oneself from bringing Ian too close with Cindy for the above to work anymore.
Well, no, because in order to tell Cindy to bring Wade you first need to join Cindy when she runs off in a snit to her preferred bar back in Chapter 3. If you skip that (and why wouldn't you if you aren't trying to steal or babysit Wade's girlfriend?), then the attempt to talk sense into Wade and Cindy will eventually fail.
And even if you do all that, you
still need to agree to chaperone Cindy when she throws her latest temper tantrum in Chapter 9, then navigate the minefield that is the Axel photo shoot just to get the opportunity to point out that Cindy's on the cusp of throwing away her relationship... assuming you also make the necessary stat check, of course.
That's my problem with Cindy in a nutshell. She's so self-centered that merely pursuing her well being isn't enough, you need to actively enable her just so she won't tune you out. Wade's made plenty of mistakes himself, but I don't need to tiptoe around his tender sensibilities to give him advice. In a game full of hot women why I would want to date Cindy (much less betray a close friend to do so) is beyond me.