I've tried to get who this guy is, but it's hard to get when there is always music playing, and they normally speak very low. Since he's there to make his documents, he must have just arrived in Poland, and that's why we never saw him on cam.
It was already a point of discussion here that Sayn couldn't make her cum. It's weir because it seems that he even improved since they started dating and Elli always liked him on bed. To me, it's strange that she's cheating on him because of this since with Peater she had worst sex than she had with Vencel. It's also strange that, is Elli loves Sayn so much, why she's doing this? Maybe she has to find a partner that accepts an open relationship.
From now on I'm convinced that Elli is just a young Masha. I guess she wasn't build to have a serious or monogamic relationship.