Hello,
Can you check and make sure that the network owner has not created a more up to date network, as he's had to reinstall hamachi on that system.
generally, if a computer is showing offline for 1 person but not others it has to do with the what version of that machine that thhey are seeing, as each new install will give a unique client id
Try geetting him to send you the network id and password for his network, and re-joining that in the application