Yup that is pretty small. I have seen different numbers on netflix but if he is say trying to watch 4k video he will max out the connection and leave no bandwidth for anyone else.
I forget how much 1080p uses but I suspect 2 people running even 1080p would max out a 15mb connection.
There really is no solution for this other than to buy more bandwidth or limit the usage of high definition video. Their might be technical ways to limit the bandwidth usage but in the end it will come down to some agreement between everyone in your house as to which traffic is actually more important and how you share it. Unfortunately some streaming devices you have no ability to set the rate it uses, it will always try to use the maximum. Maybe there is some way on a netflix account itself to limit it.