That depends on what that help is. It's illegal for the campaign to accept contributions or anything of monetary value from a foreign source. However, if Putin calls up his old buddy in the White House and says I've got some good shit on Biden. That's perfectly legal. If a deal is struck, then that can be illegal. If that shit helps the candidate raise money for the campaign, then it can argued that's its illegal. In other words, it can get tricky.
The law is 52 U.S.C. 30121 and it clearly states the following, notice it says directly or indirectly money or other thing of value.
52 U.S. Code § 30121 - Contributions and donations by foreign nationals
(a)ProhibitionIt shall be unlawful for—
(1) a foreign national, directly or indirectly, to make—
(A) a contribution or donation of money or other thing of value, or to make an express or implied promise to make a contribution or donation, in connection with a Federal, State, or local election;
electioneering communication (within the meaning of
section 30104(f)(3) of this title); or
(2) a person to solicit, accept, or receive a contribution or donation described in subparagraph (A) or (B) of paragraph (1) from a foreign national.
According to the law it doesn't have to be money and they don't have to actually take it. It just has to be offered.
So it's really not that tricky and from the language of the law, trump and his campaign broke that law in 2016 and trump said today he would break that law again for the upcoming election.