Campaigns are strictly forbidden from accepting things of value from foreign sources, and the law is explicitly written to make clear that doesn’t have to mean something with a dollar value.
What was “fairly standard” for campaigns prior to Trump was to run to the FBI with their hair on fire whenever mysterious dirt showed up in their inbox. The Gore campaign did it in 2000 when someone sent them Dubya’s debate briefing book, because the alternative was risking being accessory to exactly this kind of foreign meddling (for all they knew at the time).
You can argue about whether it’s a good or bad law in the abstract, or whether Don Jr. might or would or could ever see charges over this, but this is the second time he’s tweeted out prima facie evidence that he’s committed a federal crime. I’m not saying this is enough for a conviction in and of itself, but it could probably get him indicted, and it may very well do just that before all is said and done.