The reaction would have been more out there but probably the same. He'd be upset that Matt was not treating Karen well and "cheating" on her, as he'd probably assume he was back with Elektra. He'd also be mad that he was letting this get in front of their biggest case and ruining it. He'd storm out of there shortly after that and be with Karen more and cut Matt off completely.
Anyways, back on the subject of "switch Karen for Foggy".... my argument for the scene is that given the tension between them, it's hard to say how Foggy would have reacted. At the very least, I feel that he would've given Matt a chance to explain. Sure, Foggy would be angry with Matt for continuing to engage in superhero team-ups in the middle of the trial, and teaming up with
Elektra, who he knows just actively damaged their case by threatening their key witness, but he wouldnt be as shocked or hurt by it as Karen, and he definitely wouldnt want to get into an argument with Matt about it in front of Stick and Elektra.
With that said, I've seen a counterargument for why they chose to have Karen see Elektra in Matt's bed instead of Foggy: it's that Karen already didnt know about Elektra
*, so switching Foggy for Karen would just be a rehash of the bathroom argument, e.g. "Foggy gets mad at Matt. Foggy tells Matt to be honest with Karen. Matt continues to make bad choices. Karen continues to be none the wiser." And it's likely there's no way that Foggy would come back from walking in on that scene if he had showed up. He makes clear that he hates keeping Daredevil a secret from Karen, so the visual of Elektra lying in Matt's bed would make him hate Daredevil even more and be the final straw for him. So Foggy would have taken Matt aside and gave him an ultimatum: "Either you call Karen right this second (or within 24 hours) and arrange a meeting, or I will tell her the truth myself." That wouldn't have been good for Foggy because haters would see him as a bit pushy and a bit of a big babysitter, but sometimes a friends gotta do what a friends gotta do.
*A thing frustrates me a lot about the bathroom argument in 2x07 is that Matt and Foggy do it in the one place where they can go but Karen can't, ostensibly because that's the only way the show can keep Karen from knowing about Elektra until she sees Elektra in Matt's bed. If they had that argument anywhere else, Karen would be a part of that conversation. Actually, forget all that, after the loss of such a key witness, I think Karen would be pissed off enough that a "men's" sign wouldn't mean a thing to her. God, how would that argument have gone with Karen participating in the conversation?