Wells Fargo is turning into a deplorable company with deplorable values and he needs to let go. He should let go of the company and let the company fail at his own rate. If he holds on he could stand to lose a great deal of money as popular opinion of the bank falls.
Buffett has more than proven his investing and business prowess in the insurance and banking industries. These are businesses in which having a margin of safety is incredibly important.
In banking, as we learned through the economic crisis and the liquidity crunch that followed, banks that effectively manage their risk -- like Wells Fargo -- can be fantastic long-term investments.
Warren Buffet owns a large position in a lot of areas, and these investments have purposes. If the economy breaks one way, he makes money on these particular investments. If it doesn't, he makes money on others. It's no surprise that some of his investments end up losing money, but they were always there to mitigate risk.
Warren Buffet made his investment in Wells Fargo when it was worth a lot less. There are problems now because of the recent scandal regarding made-up accounts, but the bank will eventually get through this. His investment will then continue to grow, so it makes sense to hold on even longer now.