bed witch refers to a males friend they are not married to yet are cohabiting with or getting together on occasion. There is no intention of making the situation permanent. From slavery days in the US to designate a female slave used by white plantation owners or white overseers or any of the owners male children. A bedwitch can be told to "get out" at a moments notice.
She is "his" bed witch. She is not a girlfriend and not married to him. She is just quietly used as a sex object. She is not a mistress either.