Obviously the first reaction to hearing that he didn't hire an agent is one of reassurance, but I tried to find out exactly what the rule book states on this. Here is what I found:
NCAA Bylaw 12.3.1 (General Rule) states that an individual shall be ineligible for participation in an intercollegiate sport if he or she ever has agreed (orally or in writing) to be represented by an agent for the purpose of marketing his or her athletics ability or reputation in that sport. Further, an agency contract not specifically limited in writing to a sport or particular sports shall be deemed applicable to all sports, and the individual shall be ineligible to participate in any sport.
While "hiring" (i.e. paying for professional services) clearly voids the eligibility, the rule above itself doesn't reference "hiring" an agent, it speaks to being "represented" by an agent. Sure, one could debate where exactly you drawn the line on this stuff, but the rule reads as though if Barnes engages in the marketing activities that an agent would do - such as talking with teams, shoe companies, or whatever else agents do - that could be construed as a violation of the rule. That said, I'm sure Barnes knows what activities would clearly be across the line and is smart enough to not botch it, but based on the wording, I don't think Biggie/Barnes necessarily have a free pass to do whatever just because of the guardianship.