Regret aversion

With reference to the CFAI 2015 Morning Exam Question 11-A, I’m trying to understand the client who has regret aversion. More specifically, when the stock price of Uno increases, I thought he would have wanted to purchase the stock to avoid the fear of missing out on gains. But regret aversion is also when a FMP would do nothing out of fear of making a wrong decision (don’t buy something that has risen in value, and don’t sell something that has risen in value). Can anyone help me understand the difference better?