If the boss will look at their IP history it may be confirmed or not.
This is easily avoided by using different VPN servers for each account, using one account on PC with landline/fiber and the other via mobile data, using one account only via Tor, using one account over public or neighbor's wifi, etc.
If the IPs are consistently the same then it proves it, but if they are different then it doesn't disprove it.
Also consider this side too...
Imagine the scenario, if he would open his account in my house then theymos could see same IP.
... but luckily it did not happen (I am sure coz S_th... did not use my wifi) so I am happy. Ask theymos please to find the IP link (if any) in between mday and S_Th please.