I don't. She knows what I'm about.
If she had a problem with Catherine being a slut, she should have mentioned it the first time Catherine dragged her into a threesome. I mean, if you need to know if your partner is a slut, that should be a big sign.
Eleanor has met, and probably had sex with Kyo, so she should know exactly what sort of person Catherine is and what she is like as a Mistress. Eleanor is also the person who tens to most see Catherine and Kyo as peers, and as having similar traits.
Eleanor knows that Catherine has an elf bimbo as a live in sex slave (because she's really no use for anything but lugging around humongous boobs).
While Catherine doesn't brag about all her sexual conquest, Eleanor has heard of enough of them.
And after all that knowledge, Eleanor asked about being Catherine's submissive. So yeah, as long as Catherine is honest and doesn't ignore her, there is no reason to feel sorry for her. Feeling sorry for Eleanor disrespects her choice to be a submissive, the more respectful course is to either help her learn to accept Catherine's sluttiness, or support her if she wants out of the relationship or wants to redefine the relationship (people do change). You can't apologize to her for being the person she knew you were when she asked to be your sub.
Eleanor seems to be a bit hypergamous too (pick one superior lover and stick with them), so Catherine's sexual conquests are probably a turn-on for her. subs often base their self-worth on their opinion of their Domme, so everything about Catherine which makes her powerful or successful also makes Eleanor feel better for being Hers.
The only way Catherine could really screw it up would be to become someone else's sub. Not just letting a friend handcuff her one time, but making that commitment to let someone else control her that then makes Eleanor's commitment worth it.
The part that worries me is that Catherine should eventually marry someone to secure her ambitions, but that someone would need to be able to accept Catherine's libidinousness.