The Mishnah deals with a case in which, at the time of the kiddushin, the father agreed to give the prospective groom a certain sum of money as a dowry, and this commitment is treated as though it had already been finalized. Afterward the father backed out of the deal, and the Mishnah's expression for this is "pashat lo es haregel".
The meaning of the expression "pashat lo es haregel":
The expression is an idiom, and Rashi explains it in two ways:
The father-in-law says to the groom in a dismissive tone: "Remove the dirt from my foot."
Alternatively: "Hang me by my foot, and I still will not give you what you are asking for."
Either way the content is the same: the father is backing out of the deal, even though he already committed himself.
The ruling of the Mishnah:
The Mishnah states that the bride "teisheiv ad sheyalbin rosha" - she sits until her hair turns white, and the groom is obligated neither to marry her nor to divorce her.
Some explain that this refers to a situation in which the daughter herself was a partner to her father's act, and she felt herself to be a party to what he did. Only under such circumstances do we deal strictly with her and hold her responsible.
The opinion of Admon:
Admon holds that the daughter can still argue: "Had I myself made the agreement about the dowry and then failed to keep my agreement, it would indeed be fitting for me to sit until my hair turns white. But my father - perhaps he is a swindler, and he is the one who told you that there is nothing. What can I do? Either marry me, or give me a get."
In summary: Admon's words indicate that we do not hold the daughter responsible for her father's act. Even if she had some knowledge of the matter, she was not the decisive factor.