I think what she is saying is that because of the close and historical aspect of their relationship as 2 couples. That telling her husband the truth would most definitely cascade into his spouse finding out one way or another. The unpredictability of the reaction and subsequent actions on the husbands part going forward is the fear not so much for her but for the bf.
Good news, the town folk seems like less of a problem...today anyway.
RE Poly tendencies; I think knowing how you feel or can handle the concept of having multiple romantic relationships is only half of the equation. The other half is the acceptance of your spouse or partner to have the same. So you have to go back to roles reversed situation and not let NRE or the prize on the other side influence your answers because once that door gets opened in hard if not impossible to close. There are plenty of threads here that can attest to that.