The "poison pill" argument by Nike is a good one regarding the NB sponsored gear. However, Nike drafted the contract with the provision allowing them to match presumably without such a caveat, so tough luck.
If Nike does not match, Boris can sign immediately and the Nike contract term was already over; no new term, retroactive to 1/1/16 would start if they don't match, it would if they did. Boris has been cautious in not signing since Nike is disputing whether or not they matched.
The real issue here to me is, depsite the fact that Nike might try to put take it or leave it contracts out there, is that Meb's brother is an idiot for letting his client sign a contract with the negotiation period after the prior term of the contract. This negotiation period should be at the 180 period at the end of the term, not the 180 days after the term has already expired. You are setting your client up to not be receving compensation from anyone during that period. Crappy representation.