Ronin006: this answer to your question "Where is US law do it say it is illegal…" is:

52 U.S.C. 30121, 36 U.S.C. 510 Prohibition on contributions, donations, expenditures, independent expenditures, and disbursements by foreign nationals 

The Federal Election Campaign Act includes a broad prohibition on foreign national activity in connection with elections in the United States. Foreign nationals are prohibited from:

“Making any contribution or donation of money or other thing of value, or making any expenditure, independent expenditure, or disbursement in connection with any federal, state or local election in the United States”

Under this law, information is a “thing of value.”

Further, the law states that “No person shall knowingly solicit, accept, or receive from a foreign national any contribution or donation prohibited by [this law].” Fredo solicited something of value when he wrote, “I love it,” took the meeting, and invited others to join him.