When a person is outside of the area he can walk (his techum, which is 2.000 amot from his residence), he is only allowed to walk an additional 4 amot (one ama is 6 handbreadths).
There was an an incident which is recorded on Eruvin 43: Nechamya the son of Rav Chanilai was so engrossed in his study that he walked outside of his techum area. Rav Chisda heard what happened and said to Rav Nachman “Nechemya, your student is distressed”.
So what should happen if a person unknowingly strolls outside his techum? The solution that is offered is that a group of people should partially surround him, thus creating a new techum in which he can walk. The opening of this human (partial) fence should be close to the original techum, so that he only has a few amot to cross over to it.
There is one restriction: these people helping the unfortunate lost soul should be unaware that they are helping. This is derived from a discussion about a sukkah wall that collapsed: using a person who is aware that he is being used for this person, falls under the prohibition of building on shabbat. But if he stands there inadvertently, it is ok.
Helping people without knowing that you are helping them is not only (at least in this case) the halachically preferred option: it also makes sure the person that is being helped does not feel bad about being helped. It is bad enough to transgress. Even if it is unwittingly. Hence the remark that poor Chenanya was distressed is relevant.
People in trouble should be helped. But helping knowing you are helping can cause new problems. It can create a sense of moral superiority, or a feeling of inaptness, guilt or shame on the side on the receiving end.
Helping other people is conditional for the human race. Not knowing that you’re helping makes it even better.