My advice... tell you know and you want to watch. After he gives her an amazing fuck offer to go down on her and lick his load from her pussy! She will love it (so will he) and start including you in some sessions later. Beats jerking off!
My wife shared with me that she fantasizes about being taken like this, by a guy she barely knows, - and conceiving this way. As you notice in the video, there's no lovemaking here. He's fucking her because he can. She isn't exactly resisting ....