Lagging, but the connection is fine?

So my gf has been lagging bad in the 800 ping range. For months now. We're on the same wifi connection, only us two, and we play at the same time, she laggs I don't, and we can't seem to figure out why. If anyone has any suggestions, we'll try them all. It's been off and on, but lately it's more than ever, and more often than before.