The following contains major spoilers for My Hero Academia Chapter 290 by Kohei Horikoshi, available now in English from Viz Media.
Quirks in My Hero Academia are the penultimate rulers of society. Whether someone is born with a superpower or superhuman appearance can define if they are destined for success, a life of villainy, or if they'll even experience discrimination. Likewise, some Quirks are far more overpowered than others, creating even more of a hierarchy in society.
One ridiculously overpowered Quirk is All For One, an ability possessed by MHA's central antagonist. All For One allows the user to steal other people's Quirks, wielding their power and leaving the original user quirkless. The stolen Quirks can be gifted to whomever All For One deems deserving, even strengthening allies with multiple Quirks at once. However, some of AFO's peers -- such as Dabi -- would benefit from this Quirk's abilities, yet they remain neglected by the villain.
Who Is My Hero Academia's Dabi?
Dabi, revealed to be the previously assumed dead Toya Todoroki, is affiliated with the League of Villains and a friend of Tomura Shigaraki -- the organization's esteemed leader who was hand-picked by All For One. After being driven insane due to Endeavor's neglectful and selfish parenting, Toya became a villain to seek revenge against his father. Similar to others in the Todoroki family, Dabi's Quirk is flame-based -- but his comes at a personal price.
Much of Dabi's skin has already been damaged from losing control of his flames at a young age, and as such is attached to his healthy skin with crude surgical staples or hoop piercings. Toya's Quirk has continued to disfigure him, with his purplish, damaged skin now extending due to the overuse of his power. His Quirk is powerful, but it does negatively affect him and is clearly unsafe to continue using.
All For One Could've Helped Dabi in Three Specific Ways
It would make sense for All For One to help Dabi, considering he carries out a lot of dirty work while the main villain plots behind the scenes. Likewise, Shigaraki is All For One's chosen prototype and Dabi works alongside him -- which should be enough moral reason for the villain to help Dabi using his own Quirk in one of three ways. Using All For One, Dabi could be given a new Quirk that counteracts the damage from his flames. Any Quirk with regenerative, healing or fire immunity capacity would save Dabi from the pain of burning his own flesh when he uses his inherited abilities.
AFO could also have taken Dabi's Quirk away from him entirely, even if the teen was strongly against it. Without a Quirk, the stress embedded in Toya from embodying his father's dream would be irrelevant, and he could live a relatively normal life; one where his Quirk isn't slowly decaying him, and he could potentially rekindle his relationship with his similarly traumatized siblings.
Finally, AFO could have used his Quirk to take away Dabi's Blueflame and replace it with an equally powerful Quirk that wouldn't cause him any damage. Again, Dabi may have been against the removal of his own Quirk, but replacing it with something else would actually separate him further from the Todoroki bloodline, giving him better means for revenge against Endeavor.
Why Won’t All For One Help the Struggling Toya Todoroki?
While essentially saving Toya Todoroki's life would take mere seconds for All For One, he doesn't appear to have made any effort to do so. The potential to use his overpowered Quirk for good was essentially handed to him on a silver platter, yet the villain refused to do something any Pro Hero would do in a heartbeat.
AFO has a goal to thwart humanity's future altogether, and will only use his Quirk if it aids this. The villain has recently gifted Tomura Shigaraki -- now the Grand Commander of the reformed Paranormal Liberation Front -- multiple Quirks including a regenerative one, but only did so to strengthen his successor and continue to use him like a puppet master. It's clear All For One doesn't mind bestowing Quirks upon his allies, but only if it directly aids his long-term plan for the downfall of humanity.
While saving Dabi could benefit All For One, he is a villain who struggles to see past himself and his own selfish benefit. Therefore, he is unlikely to extend a helping hand to the eldest of the Todoroki children and will continue to use Dabi until his Quirk renders him useless.