That said, B's financing G is niether here nor there in this context , even if B was indirectly financing J ...
1)B's help had come to G in a different context where he had financed her education etc for a very different purpose , not to fund her live in relationship with J- he might not have stopped it for sometime after she got together with J but that does not imply that it in context of her being kept by J
2)Also B and J are not one and the same person here- the derogatory gendered aspect of being a mistress comes from a direct relationship where the man is a master who provides and keeps a woman living with him with whom he has a sexual relationship. Niether in terms of the original reason for giving money to G nor in terms of the man who lives with her , does this financing by B fit the situation. A father's wealth is not the same as the son's wealth unless given to him specifically by the father - and J had been disowned by B
In any case , i didnt mean to go on quibbling on something , but it does offend mine and perhaps some other people's sensibilities when G is labelled a mistress in what is meant by the person using it to be a clearly derogatory expression.