If broker dealers give investment and/or rollover advise to IRA or self-directed solo 401k account holders, it is treated as advice to a retail customer and is therefore subject to the SEC’s Regulation Best Interest (Reg BI) requirements.
The reason recommendations to solo 401k plan are covered by Reg BI is because a solo 401k plan is an owner-only plan. Recommendation types include how to invest the solo 401k funds and, more importantly, rollover recommendations (i.e., to rollover the solo 401k fund to an IRA).
Here is what the SEC says:
“Best Interest to expressly apply to account recommendations including, among others, recommendations to roll over or transfer assets in a workplace retirement plan account to an IRA, … and recommendations to take a plan distribution for the purpose of opening a securities account…”