A very dear friend of mine is a congenital LBE (left below elbow) amputee due to a birth defect. I want to make a sim of her that properly reflects her real self. She doesn't wear a prosthetic arm very often so I didn't want her sim to have one. I understand that in game this would look odd because the sim would pick things up and it would just float there, but Sims use their right hands more anyway...

I've seen a headless skin before and I've seen missing legs so I know it's possible. I was hoping that it could be made as an accessory (glasses) so it could be used with different skins.

This couldn't be an accessory, it would have to be clothing (since the arms are part of the sim's shirt mesh). Hopefully someone will have seen such a thing.

I've had a look around for you but couldn't find anything sorry. There's a glitch that makes the whole of a sim's arm disappear shown here and here
Someone has requested a full arm amputation over at GOS here

You might have to request it from other creators.

Theres a request thread on MATY (you have to be a member)
There's this thread that shows creators willing to take requests.

or there's GOS which also takes requests.

Sorry I couldn't help.

Those sims' arms aren't actually missing, though - they're just poking out the wrong side of the sim's body. The fishing sim is holding the fishing rod in her right hand and the book in her left.

For the requests, would that come under meshes?

Yes, it would.

I tried to register for MATY but it keeps on telling me that I look like a robot so I can't register.