Strange Latency Issue

Sorry for the wall of text. I'm a cable technician, and for the last couple of days, I've been working with a commercial customer troubleshooting a latency issue. I can be fairly certain that the issue is not RF or plant related. When troubleshooting, I connect my laptop to the modem (a gateway device with multiple ethernet ports) and ping Google. My ping times would be in the 100-300 ms range, until I disconnect the customers network from our modem. Then my ping times drop to normal, 30-40 ms. So, customer network issue, right? I figured case closed after that. They're an IT company, after all, they can just fix whatever is eating up their bandwidth. But upon further troubleshooting, we left their network disconnected, and hooked up the customer's laptop in addition to mine and the customer starts downloading a file at maybe 10mbps. They have 50 meg service. The ping times shoot up again. The customer is also running a traceroute utility, and it's showing the ping times jump to the high levels between the first hop and the second hop (10.1.10.1: modem, and 96.etc.etc.etc.: our CMTS). This is a MacBook pro, by the way. We do the same thing on a different MacBook Pro, with the same results. It seems that with any minor activity on their network, ping times go crazy.

It's very doubtful that this is an issue with our cable plant, as our line techs and head end have looked at any possible RF issues in the area, as well as potential capacity problems, and neither is currently a likelihood. Also, the cable drop and outlet were bypassed with a test drop straight to the tap, all with the same results. I even had their 50/10 meg plan bumped to 100/20 temporarily to see if that alleviated the issue, with no luck.

However, I removed the customer laptop, left their network equipment disconnected, and simply had my laptop, my work phone (iPhone 7) and my personal phone (galaxy s8) connected to the wifi on the modem. I ran a Netflix stream on the laptop and YouTube videos on the phones, and ping times stayed quite low and steady, only jumping a bit when things were buffering. I also did a hangouts video chat between the phones to try and tax the upstream as much as possible, but still no significant blips in ping times.

It only seems when the customers Apple computers are connected (most of the rest of the machines in their office are Apple computers, and no I'm not an Apple hater), do ping times go haywire. I've asked them if they have anything running that might cause this, such as icloud backups or anything else, but they swear they don't. I'm not entirely certain what's going on here, and if anyone has any ideas of something else to try, I'm all ears.

Thanks!