This is not really the forum to get a full rabbinical education,but the short answer to your question is Ein davar she’b’ervah pachos mishnayim.
In the rare circumstance that we allow a lady to remarry based on an Ed echad, there are a number of criteria that must be met, including the woman testifying that “bari li” among other criteria.
The way you are asking the question indicates that you aren’t a major masmid in BMG,so I am going to assume that you are a BY graduate. (not that it makes a difference to the validity of your question)
In general עד אחד נאמן באיסורין will be in effect with a minor issur (think not chayav kareis) where there is no financial incentive (think a shopkeeper on his own product).
Again, this is the light version of the answer and to really address the question you must take steps to learn more. please find a shiur on the topic, rather than putting your genuine curiosity to the whims of random (often unqualified) opinions on the internet.
And no, while there are many bnei yeshiva in the coffee rooms of yeshivos far and wide, the coffee room is not the appropriate place to get Torah education.