Friend's Warframe Gets Bugged In-game

[Prism77]

When I am the host of a game and my friend joins me, his Warframe disappears about 5-10 minutes into a mission and all that can be seen is his username off in the distance. He also said that he was unable to interact with anything and doors wouldn't open for him. He can only interact with objects and open doors in his Operator mode. I'm not exactly sure what is causing this or why this happens, but it most commonly occurs when it's just me and him in a mission. Here's an example of what he appears as from my perspective: 

As you can see by his username he is off into the distance out of bounds. Very strange indeed.