Explanation of The Book of Divorce: Volume 3

Thus, the presence of a third person negates their being alone, but this
third person must be sensible and vigilant enough so his presence can
be a guard against temptation, or else evil would be done while he is
unaware. This is why the third person cannot be a little child, even if
he has developed some perception [i.e. as of seven years of age] as this is
not enough to negate the meaning of “alone with her” unless he is an
exceptionally smart child who is keenly aware of what goes on around
him. Otherwise, a seven year old child is not enough, for the man can
give him anything to play or stay busy with, and then he can say or do
whatever he wants. Thus, there must be a third person in the company
whose presence prevents evil. A ten year old child is apparently enough,
for he has started to develop good perception, but he may not be enough
since some children at this age may not understand this.
If a man is with a number of women, this is not a state of being alone
with a woman, for he is not with one woman only. If he is to be trusted,
it is permissible to be the only man with them. Likewise, if two or more
men are with one woman, there is no “man and woman alone” in this
case. However, deeming this company impermissible (that of two men
and a woman) is not groundless, especially if they know each other or
are known to be like-minded. If they are not, like a taxi driver and a
complete stranger, and the place is safe, it may be deemed permissible,
though I see it worthy of reconsideration. If a woman rides a bus alone,
or any such kind of transportation means, it is impermissible, though
this is not as serious as a taxi. Public means of transportation are at least
governed by stations where they take off and stop. Still, no woman is
permitted to get on a bus alone, but two women at least or a man and
a woman must be together on board to prevent her being alone with
the driver.