I'm a woman in my mid-twenties, and a male friend of mine wants my help to put together a grand, romantic gesture to convince his crush to date him instead of going back to her cheating boyfriend. I think it's a bad idea, and I want to discourage him. What should I do, and what should he do?
The phrase, "He's just not that into you," took off for a reason. If two people want to be together, barring other extenuating circumstances, they usually find a way to be together. If the feelings are one-directional in either way, they usually don't get together, at least not for a substantial, high-quality relationship based on mutual respect. Feelings can change, but they don't usually do so based on rose petals or sky writing. These things only work in romantic comedies, or for couples where both members think they're in a romantic comedy. It's just not how real life typically happens.
I actually think romantic gestures have a great place in relationships. When two people are already in love, illustrating that love on a large, bold scale is sweet and touching and can help the relationship blossom or recharge. But in this kind of pseudo-courtship, your friend would be putting himself in a sad and vulnerable position, and even if she decided to be with him, the relationship would be based on him wowing and bowing to her in a way that wouldn't put their relationship on strong foundational footing.
He could be a good friend and sit her down to express his concern that she's considering such an unhealthy romantic relationship. If he couches this intervention in "choose me instead" terms, he diminishes the seriousness of the rest of the conversation. If she won't listen to him for whatever reason, he should find someone to talk to her whose opinion she'll take to heart, but again, about the cheating boyfriend and her own need for self-respect, not about other potential prospects.
Another wrinkle here is whether he's already expressed his feelings to her and is considering escalating that declaration, or whether this grand gesture would come as a complete surprise to her. If he's never expressed how he feels, there may be some delicate and appropriate way for him to say something like, "There are good guys out there who would love to make you happy and would never cheat on you, you know, like me." While that line could, admittedly, be taken from a bad romantic comedy, if she doesn't have any inkling that someone else could care for her in a way she deserves, knowing that actually might impact her decision.
You can pass this advice on to your friend. You can tell him you support him, but you won't help him make a fool of himself. Then you can back off and leave the decision in his hands. Hopefully he'll decide to be a good friend and put his feelings on hold in respect of this woman's best interests.