Short summary of context: Something happened with 兵太郎君 (heitaroukun) in the river, and his friends were there. The speaker is one of the friends, 久助君 (kyuusukekun). 兵太郎君 (heitaroukun) didn't show up in school ever since. Now the speaker asks himself why that is, or rather, if they (kyusuke&co) are the reason, Heitarou isn't showing up anymore.
I get the vibe of the above sentence somehow, which in my mind looks like this in English:
But, if in fact Heitarou got sick because of the river-incident, there wouldn't be any reason for him to shut up about this.
Question: How can I parse もとでじっさい in that sentence? Did I understand the sentence correctly?
What I researched on もとで:
下で: From a german/japanese dictionary: under, by, in. (I don't see the sense here if that would be it) 元で：I didn't find 元 with で, but I guess combining them the meaning becomes In fact(?). But then why is there じっさい behind it?