It's ok to ask what she prefers. In the meantime, your default should be to let her finish when she's in the middle of stuttering or a block or a prolongation. If she gets blocks a lot, I'd ask her if she can think of some of her secondary behaviors that people should know about. Frowning, raising eyebrows, eyes widening are secondary behaviors that should be disclosed so that people don't interpret them as meaningful non-verbal signals.