So if he is not actually fucking her (a big IF) he is the good guy than? (aside from being a host that is but he mentioned she doesn't actually have much money so he is not there to bleed her dry) Which make the MC an actual nuisance in this case considering he is not equipped to deal with this, like at all. He also has his own considerably heavy baggage. Just like in the previous story where the girl wasn't and almost fucked it up completely.
If we assume, with a huge if, the host dude isn't visiting her for sex, but is actually her relative, with perhaps his parents pushing him to look after her, I reckon he would be extremely glad to have someone else share the burden of taking care of Kanon. So, in such a case, the MC wouldn't be a nuisance but a big relief for the host guy. He doesn't really look like a saint who would out of pure selflessness try the keep helping a mentally ill relative most likely for years.
Since the dude is a host club employee, he would be an extremely competent liar and bullshit talker, because that's essentially his very job, so the MC isn't going to beat him in a duel of words. Perhaps the MC can make Kanon spill the beans. Who knows. I doubt he has what it takes to really interrogate her.