I had this problem at first too. Your IL 2 server is behind your home firewall so your friend is being blocked from connecting to it. You have to setup your router to forward the port being used by the IL2 server that your running. Check the server documentation for the number.
I ran into this for the first time today (March 1st 2020). It happened after I had been playing several other graphic intensive games. So I think it may be a graphic card memory issue. I rebooted and was disappointed to see that the problem persisted on the p-51 in the hanger. However, when I played a quick mission, the issue was gone, and then the glitch vanished from the hanger p-51 as well.