It's hard to say for sure. Deku has grown a lot, but All For One is a powerful villain. It depends on various factors like their current abilities and the circumstances of the battle.
Deku might be completely shocked at first. Since Bakugou has always been so aggressive towards him, he would never expect such a confession. He might just stand there, mouth agape, not knowing what to say.
In 'All for One Deku' fanfiction, it's interesting to explore the new relationships Deku would form. For example, his interactions with villains who were once on the opposite side of him. Also, the power dynamics shift. Instead of being the underdog hero, he could potentially become a more complex and morally ambiguous character with All for One's powers. His internal struggle between his original self and the new, darker influence can create really engaging storylines.
He might cry a lot. Deku is very emotional, and being cheated on would be a very painful experience for him. He could also get angry, which is rare for him, but this situation would be so unjust that he might not be able to hold it in. He'd probably start to question his self - worth too, thinking that there was something wrong with him that made his partner cheat.
Deku might be initially shocked. He's used to Bakugou's aggression, but an actual injury is different. He could be in pain and disbelief at first. Then, he might try to downplay it, especially if he doesn't want to cause trouble for Bakugou, because Deku is generally a very forgiving character.