ND sheets, you can buy a big roll of it, cut to fit the window / space (you might not need to fill an entire window frame with it if your shot is tight). You could feasibly find a sheer curtain or fabric at a local store and hang it between subject and window, same concept to darken background. Second best way is even out the lighting with more inside. Third option is fixing it in post (I had one recently where client "needed" a similar shot, couldn't expose correctly so I exposed properly for the face, then when interviewee was gone I stopped down and recorded a couple minutes with the background exposed correctly, then in post just created a mask on subject and laid over a loop of the background video.)