I've been seeing posts about stuff like this lately and what I haven't seen enough of is clear statements about:
The point of kashrut is not that something will happen. That's a Christian framing. Christianity posits sin as something that has a direct effect. Judaism doesn't.
Kashrut is centered around the idea of intent. It is a practice. Feeding an uknowing person who holds by kashrut (at whatever level, in whatever way) food that violates their practice does not unleash some kind of wrath of God on them. It is, however, a violation of their trust, and of their practice. It has self-imposed consequences. The entire premise of "I will feed this person a thing they refuse to eat to show them that it's fine" is not only unethical, but it has nothing to do with the cause/effect of that person's choices and perspective religiously or culturally. (It also should be unnecessary to point out that the consequences of violating a dietary restriction are often not immediate, and feeding someone a food they aren't used to eating can cause problems many hours later.)
Judaism approaches the commandments and the mitzvot (or religious obligations) from the perspective of those that are between man and God, and those that are between man and man. Kashrut is between man and God, ie. each individual has to make decisions for themselves and those decisions are between them and God (or not, in the case of atheist Jews), and no one else. There is no place for another person to intercede, and if they do, the consequences will be in the affected person's conscience and emotional soul. (Which also means that from a Jewish perspective, these in-laws were positing themselves as God, which I'm pretty sure is also not something Evangelicals are fine with, let alone Jews.)
When it comes to kashrut, like I said, the framing is centered on intent. For example, kashrut requires leafy greens to be checked for bugs, because bugs are not kosher. If a person does their due diligence to check for bugs and finds none, and ends up eating one that they unintentionally missed, they have not violated kashrut. However, if a person does not check for bugs and eats a leafy green, even if it has no bugs on it they have violated kashrut because they didn't check.
The thing is, an example like the above AITA is not in violation of kashrut, but the person affected nevertheless felt violated themselves, and likely guilty and possibly tainted. And while they don't have to, a lot of people in this situation still do feel this way, and that's natural. Many rabbis will say that to resolve that guilt you can do teshuvah, whether it's through davening or tzeddakah or both. (I think Chabad.org has a page on this specific issue but tbh I don't feel like linking to them for a number of reasons so feel free to do your own research or talk to your friendly neighborhood rabbi.)
So for any Evangelicals who want to feed Jewish (or Muslim) people food that violates their religious practice just to prove a point, maybe just cut out the middleman, don't mess with people's food, and donate to your local homeless shelter instead (but not the Salvation Army).
