Person 2 is being dreadfully picky in my opinion. For one, to say that something "sounds very ghetto" as a negative is pretty insulting to anyone who lives in a ghetto. There are certain racial and social undercurrents to this statement that are far too complex to go into here as well. To get an idea for yourself, google "ghetto speech" and you will see a wide range of interesting material, as well as a good deal of hateful and racist commentary.
Suffice it to say that "where is it at?" as opposed to "where is it?" is somewhat "slangy" but also commonly used, at least in American English. Whether or not it is technically grammatically incorrect, it is common enough that most people just don't care.
Also, the informal idiom where it's at generally means a fashionable place where the most interesting things are happening at present. By extension, it can be used to refer to people, things or activities as well:
Studio 54 is where it's at.
Whole wheat spaghetti is where it's at.
Kant's Critique of Pure Reason is where it's at.
And so on.
First, understand it's somewhat weird to have a conversation where A asks B something, B accepts, then A immediately questions it. So this means there are things going on that are not being explicitly talked about.
There are many reasons in language to "indirect-ify" things - examples include telling superiors something sensitive, simply being polite, business dealings, taboo subjects, trying to get someone to take a hint, etc.
Modals, particularly would, should, could, often are involved in these situations where you want to say something, but can't be direct due to some social propriety, either real (in the case of formal meetings, etc.) or fake for humorous/sarcastic effect. Used wrong, modals can make you sound snobbish.
So, your example:
A: I'm going to the cinema tonight. Do you want to come along?
B: Yes, I do.
A: That would be very interesting.
What I gather from this conversation is A did not take B's acceptance seriously at all. It's an indirect refusal. It might even be A snubbing B - maybe A didn't really want to invite B, and B expected A to politely refuse. This is a type of conversation that would probably make much more sense spoken where you can gauge the mood and emotion involved a lot better. I can imagine A laughing at B's acceptance, and then saying "That would be very interesting."
Now, this:
A: I'm going to the cinema tonight. Do you want to come along?
B: Yes, I do.
A: That will be very interesting.
Here, something will happen if B goes to the cinema, and A knows it. A was not entirely sure if B would go and believed that he likely would not. Perhaps B's ex-girlfriend is going to be there. Or someone else that A knows B wants to avoid. However, there is no snobbishness here. A is simply making a comment about what B wants to do.
Hopefully that makes sense. Would is definitely more polite in most situations but it can be used to opposite effect as well.
Best Answer
Will is ordinarily not used in if clauses to signify mere futurity. It is used only in a handful of situations:
If it is employed in the sense of be willing (to VERB)—or, in the negative, refuse (to VERB):
If it is employed emphatically with the sense of insist on VERBing—again, the negative has the sense of refusal:
If it is employed to signify that you accept a prior prediction as fact—here the negative implies a negative prediction:
Consequently, unless what you're speaking about involves one of these situations, you want to use do:
—But as the commenters point out, we're more likely to use BE going (to) here than bare do.