When my partner connects to my internet, he has lag spikes and loses connection

I have a 100mbit connection and I never lag out on any of my device (rarely on wireless). My internet is wired through PS4 and PC, wireless on phone. He brings his laptop when he visits me so we can play WoW together, but the problem is he very frequently gets lag spikes. This never happens on his own network or any other Wifis (he travels). At first we thought it was because he's on wireless but when we used my PS4 cable the issues remain. Does anyone have any idea what can be done?