He is going to do something about it, that's for sure. Probably fighting it. Because he is, again, astoundingly stupid and what are physical boundaries, they sound delicious.

...yeah seriously she is fortunate he's both taken and...mostly monogamous, fun activities after partying notwithstanding. Totally not cheating if your boyfriend is present/participating, however. He likely is going to want to say something to her after fourth-wall, however, even if it is just verifying whether he hallucinated that or not - he's also one for being, for lack of a better term, a thundering dork regarding magic and whatnot from other words. Because his abilities center around an ability to sense the way energy flows through things both living and inanimate, to say nothing of through the air itself, I can just imagine what the usage of a Noble Phantasm would look like in that regard, goodness lord.

So yes, basically I highly approve of Saber blowing his mind all you want during fourth-wall, he will be highly receptive and will probably geek out at her afterwards while trying to retain his composure because He Is A Proper Gentleman He Promises.