Why would it not be legal? The “marraige” he had in Kenya was a tribal ceremony and not a legal one. If I have a ceremony of a wedding and never turn in a marriage license application or get it formalized by the state, I am free to get married without divorce.
The marriage in Kenya was legal in Kenya, in Britain, and in the US. Therefore, he could not get legally married in the US.
Had he taken Ann to Kenya to marry her there still would have been legal issues concerning that marriage as she was white, he was black, and he already had a Tribal marriage.