Why would he? If she can do deals that are too small for BRK, the gains won’t be material. If she were to compete with BRK he doesn’t have a reason to invest in her vehicle either.
Where did you guys see this?
His personal account is way smaller so I would imagine he would invest with her if he thought she would do better. Look at Munger and Li Lu.
It was in the stories the day of the announcement. From WSJ: "Mr. Buffett said he understood Ms. Britt Cool’s decision because he also left a job he loved, at his mentor Benjamin Graham’s investment firm, to work for himself in 1956. He added that he doesn’t intend for Berkshire to invest in Ms. Britt Cool’s new venture."
I think he may have said that more reasons than that he didn't think she would succeed. First, he didn't say that he wouldn't invest, but that Berkshire wouldn't invest. We have no idea what he does personally; my guess is that he does not invest in other people's funds based on his history. Can anyone recall a single instance when he did that? He has never been one to outsource his personal investing. Second, if Berkshire invested in Britt's new vehicle, that might set a bad precedent. Buffett doesn't want to give an incentive for people to leave Berkshire, and he probably doesn't want to be in a position of having people leave and investing in some but not others.