IRMA surcharges – how to determine the surcharge for the year

My dad switched to a new advisor about a year and half ago. He originally had a fixed index annuity that we decided was a poor investment and decided to take the penalty and cash it out to invest with an advisor. in addition to the annuity, he also had a significant chunk of a blue chip stock that he got while working that was worth about $350,000 in total at its peak late last year. In the recent months, the stock has taken a sharp hit and that position is down about 60k.

The reason why I bring this up, is the advisor noted the significant concentration and we all agreed to sell it around October 2025. As of this February, the advisor did not sell any of it, and when I asked, he said he didn't want to trigger an Irmma charge. my dad's MAGI for the past few years was in the upper 90s due to him taking RMDs.

When we started talking about this, he explained that if he fell into the first Irmma bracket, he would have to pay a 202.90 Part B base premium along with a 82.20 surcharge per month and would have to pay 284.10 on top of that since he would be in the 1st bracket of Irmma surcharges. He explained that people making over 500k would pay almost 27k in premiums+irmma, which made no sense because I would think you just would opt out of Medicare at that point. I thought it was just the base premium+ the surcharge only for Part B. If so, we would have been fine with taking the 1kish hit to sell about 40k of the stock. If the hit was 3-5k, we would not.

How does Irmma work and is the advisor wrong?