It isn't a universal rule not to suggest the word for them. For example, I generally like it since it just saves me a bunch of effort and gets the conversation moving again. So asking what they want is the best thing. I think just listening patiently is the best thing. I generally don't make eye contact with people whilst I'm stuttering, but it means a lot when I look back and they're still listening - as opposed to suddenly finding their shoelace or tea mug so fascinating. Sometimes I even have people get out their phones or a few times even just start a new conversation with someone else, it's incredibly rude and really hurts, particularly the latter.