Asking if someone has this disability or that disability may or may not be rude, but asking a disabled person if they want your help is ok. Also, asking "how should I help you?" is ok. As for finishing sentences. Level one advice: let a stutterer finish. Level two advice: let a stutterer finish unless a stutterer indicated otherwise verbally. I specified 'verbally' part because people are bad at reading non-verbal communication of disabled people. it's like, don't stop a marathon runner. He's running and he wants to get to the finish line. It's hard, but he wants to get to that finish line. His face may be like frowning a lot. Maybe it's the sunlight. Or maybe that's just how he pushes forward. So his face is like that. And he wants to finish.