Even if she were a Russian agent, it’s perfectly legal for Jr. to meet with her. The only issue is that anyone working for a campaign would have to report such a meeting. But that only applies if the person working for the campaign knew that the person was a foreign agent, or if the person claimed to be a foreign agent (even if that claim were false.) That doesn’t apply here.
It could show willingness to collude with Russian govt. Is that a crime? Dont know. Turley points out we dont use the Logan Act