Who cares what the age difference was? That’s not the immoral part.
So you’re OK with young women marrying old guys that are an inch from death...just for the inheritance?
That is essentially what she did by sleeping with him.
She did it for her career.
The fact that he was old and very unattractive — to any clear thinker — just proves that she was exploiting a sexual relation for her own advancement, which is essentially a form of prostitution.