The fact that he reinvested his RMD into a normal brokerage account has no effect on its taxability.
You may be confusing this with a "rollover". A rollover occurs when you withdraw cash or other assets from one eligible retirement plan and contribute all or part of it, within 60 days, to another eligible retirement plan. You cannot "rollover" an RMD.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
