Happosai decides to decide to go look for Akane at the moment after looking around Nerima seeing that he was the only human apparently that was around. At least to his knowledge as all around he merely saw gargoyles or the rubble of what was once a gargoyle having been destroyed by several various means.
Thinking that possibly Akane would be trying to return home after what happened. He was sure that she had managed to make it though what was happening in one piece or was merely hoping that was the case which he found to be true. Though what he found was her...and Ryoga defending her against an attacker. The lost boy was seemingly not easily winning against them. The grandmaster of Anything Goes could see if they were still human or not given how they dressed.
Though the question of why Akane was not with her sisters was something he wondered as that a thing he was sure would be the case. Akane would make her way back to the Dojo once she was able to do so to check on her sisters herself. Of course, the reasoning for why she had was pretty clear as they must have taken her after the sun rose, and then she would not have any way of defending herself against being taken from her sisters. After all, how could she if she was currently a statue as the only thing she could do at this point is merely stand there in place on her own.
However, it seems that the reason she was fully taken away was because of the mess that Nerima was in at the moment as that would make sense given that no one would be able to move around a statue the size of Akane around that easily. She must weigh nearly half a ton or more in her current state.