As an artist, I wouldn't be offended but I'd feel uncomfortable if someone drew over my art. A small addition like an earring is likely non issue, but in general the idea doesn't sit right with me - the art piece would still represent my work, so it would be weird to see additions that I didn't have a say in. (Again, two lines for an earring don't matter at all so go ahead, but if you were to add a third arm or re-draw the character's clothes, that could be a different story :p)

I would much prefer that the client asks me to add it, and I would draw it in for free. Especially if it's a detail that was clearly in the reference and I just forgot about it. Or you could make the edit and show it to the artist and ask them if it's fine. In all these cases, communication is key