Unbearable lag but only when playing with a certain someone

So I'm playing a lot of Dota 2, and everything goes smoothly on all of the games I play. 200ms ping max and no packet loss, which is pretty normal as SEA is full of water. But when I'm playing with a certain friend, I constantly get 1000ms ping and up to 90% packet loss. It happened twice a month ago and I chalked it up to my internet connection. Before then, no problems when we were playing together. Since then, every game I've played without him has been smooth again. I've even been playing with other friends on the same party. And then we played together today and the same thing happened.

Any ideas on what could be the cause of this? He's not having any problems, only I do.

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