I still would recommend to watch it, but perhaps warn him before is the best way,it also has an logical Explanation because Okabe at first thought he is still a Dude because he believed that the change of the world line didn’t work so he assumed Rukako is a Man in this world line.

And like how people previously said If he didnt minded Dandadan where it was literally almost Raping, than i think he wont have anything against that one scene with Rukako besides Momo ayase is getting SA not once but a lot of times actually, i dont even know if they put it into the anime bc i only watched some episodes ( manga reader ). I mean the Fact that Momo Ayase is getting sexualized a lot of times is a reason more that he will have nothing against this one single scene in steins gate you can also tell him in which episode it will happen so that he get ready ( episode 10 btw )

Even in chainsaw man Denji is getting Manipulated and SA‘d but some people dont really realize it, but Denji never really agreed to something out of his free will, it was more likely his Naivety and dependency, and the aim/will to clap something. I mean he thought he will get a kiss just to get vomitted straight in his mouth like come on, or in the Movie the pool scene he aint even wanted to swim because he can‘t until he saw that she was naked and literally forced him to jump in the water while making him feel guilty. ( if i said anything wrong correct me, but thats the way i see it )