If this is not happening in your house, let it go.
They are adults who make their own choices and it is actually not your concern how they choose to live.

SD is at GP and not with you,  She earning a living, you are not supporting her. 

If hubby knows she’s so messed up beyond help, let it be.

she’s happy to be a hoe so be it

What exactly do you expect your husband to do?

The grandparents are free to decide who they allow to live in their home.  If they are fine with her living there rent free, it's none of your husband's business.

  If a grown woman wants to have sex for money, that's her business.

Nothing you posted impacts your home, your finances or your lady parts so it's really none of your business.

There is nothing in this that you can control. There is nothing in this that DH can control. All involved are adults. Adults make their own choices (no matter how poor they are). As long as no one is asking you for money to support this, do as husband does and ignore.

Are you more upset that your SD has the ability to live rent free as a full grown adult or that she has an “immoral” career? Either way, just let it go. It’s spring time and lovely. Why not use your energy to plant a nice garden or take up a new hobby?

Not your house - not your ho - not your problem.

I would be worried about the SD getting her in trouble.  Yes, agree with Petro, would try to protect GM.  There could be serious legal problems for GM

If the SD's maternal GM, not my circus, not my monkey

Shes been an adult for nearly 2 decades. What else is there for your DH to do but to detach? How old are the grandparents and are they all there mentally or being taken advantage of? 

Your profile states this SD36 is your Dh's SD from his previous marriage. 

I suppose the grandparents(I assume your DH's folks since it seems to frustrate you so much) have tarken to and accepted the child as their true granddaughter. But yes, even with that consideration, I think it would still bother me to some degree that this grown woman is taking such advantage of the older grandparents.

Bright spot is , though, that the SD is not trying to sponge off of her father and you and her selling herself out of cash isn't taking place anywhere that actually affects you/your home.

.

Listen our Seniors and children should be protected. IF you believe Granny is being taken advantage of, call Social services.

Reminds me of that old joke...

Haven't I seen you somewhere before?

...do you watch porn?

They are all adults, what is he supposed to do? He can't tell any of them to stop their behavior. I'm not clear why it's so upsetting to you.

Ignore the whore (literally a whore in these circumstances).