@jak I assume MC believes that since she cared and healed Ale back then, he'll now have the inner strength to get over his future breakup with the novel's heroine. MC thinks the heroine's closeness to Ale in the story caused his breakdown when she "betrayed" him for the prince; since heroine's not close in this timeline the breakup will less likely result in Ale's ruin.
@zDarkness I've seen some otome manga/novels that kind of justify the MC's fear/reluctance of changing the story due to overrriding influence of Fate/Source Material direction - no matter what they change, the source material will correct itself so the major event happens but in different times/circumstances. But since there's no evidence of that here, then all this fear is just in MC's head.