[pr0crast]

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Can we be sure he is the only one USING that IP?

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No, we can't. It could be anyone else with access to his home cable modem.

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Do you know that it's his home modem for sure? I know that it is residential, but I don't know for sure it's actually his.

It seems like we've only confirmed that it is:

a) residential (someone told me this) and
b) something he used (you confirmed this in your earlier post)

I think it would be his home, but don't know for sure.

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Check his ISP's TOS. Most of them are going to have a section in there that explicitly forbids a bunch of activities, and if you can prove that he broke the TOS, the ISP might be more willing to divulge info.