A woman who is well-known for lying testified that she breast-fed the one who wants to propose to her; should her word be accepted?

The scholars differed as to whether breast-feeding can be
proven by the testimony of one woman. The majority are of the view that
breast-feeding cannot be proven by the testimony of one woman, and they
differed concerning the number required to prove breast-feeding. 

The Hanbalis are of the view that breast-feeding can be
proven by the testimony of one woman. They quoted as evidence for that the
report narrated by al-Bukhaari (5105) from ‘Uqbah ibn al-Haarith (may Allah
be pleased with him) who said: I married a woman, then a black woman came to
us and said: I breast-fed both of you. I went to the Prophet (blessings and
peace of Allah be upon him) and said: I married So and so, daughter of So
and so, and a black woman came to us and said to me: I breast-fed both of
you, and she is lying. He turned away from me, so I came to him from the
other side and said: She is lying. He said: “How could you keep her as your
wife when that woman has said that she breast-fed you both? Leave her (i.e.,
divorce your wife).” 

The more correct view is that of the Hanbalis, that
breast-feeding may be proven by the testimony of one woman, because of the
hadeeth quoted above, on condition that she be of good character and there
are no reservations about her testimony. 

Ibn Qudaamah (may Allah have mercy on him) said in al-Mughni
(8/153): If one woman testifies to breast-feeding, then marriage [of the two
who were breast-fed by the same woman] is haraam, if she is of good
character. End quote. 

Al-Bahooti (may Allah have mercy on him) said in Kashf al-Qinaa‘
(5/457): If one woman who is of good character testifies that she breast-fed
him five times within the first two years, or a woman of good character
testifies that So and so breast-fed him five times within the first two
years, or one man testifies to that, then breast-feeding is proven thereby
and there is no need for an oath to be sworn by the one concerning whom
testimony was given or by the woman who witnessed it, because of the report
of ‘Uqbah ibn al-Haarith (may Allah be pleased with him) who said: I married
Umm Yahya bint Abi Ihaab, then a black slave woman came and said: I
breast-fed both of you. I went to the Prophet (blessings and peace of Allah
be upon him) and told him about that, and he said: “How can you (marry her)
when she has said that?” And he forbade him to marry her. According to
another report he said: “Leave her (i.e., divorce your wife).” Narrated by
al-Bukhaari… And the testimony of a woman who is not of good character is
not to be accepted. End quote. 

Based on what is mentioned above, that the view of the
Hanbalis is most likely to be correct, with regard to this woman who is well
known for lying, her word is not to be accepted when she claimed that she
breast-fed that girl, because she is not trustworthy. 

But we also advise you to be cautious in such matters and to
look at the circumstantial evidence. If it is possible that what she says
could be true, such as if she had a baby around the time when she claims
that she was breast-feeding, and such things were common among you (i.e.,
women breast-feeding one another’s babies), then to be on the safe side, not
going ahead with this proposal is a good idea, especially as it is the
matter of a proposal, and if the man does not go ahead with it without
giving a reason, he is not doing anything concerning which there are shar‘i
reservations, and there will be not be any trouble or problems. 

But if it is the matter of annulling a marriage contract that
has already gone ahead, then it should not be annulled on the word of
someone like this woman. 

And Allah knows best.