OP hasn’t elaborated though in terms of the extent of the lie.
If She’s just referred to her “daughter” in terms of it being a stepdaughter who she sees as her daughter then it’s just about her take.
But if she’s invented a whole life, where there’s a daughter, and they do things together and take talks about when she was pregnant etc that’s a whole other level of deception.
There isn’t enough detail to say how bad it is.
What has she said about this lie, now that she’s been found out?
Someone who makes up a whole life like that isn’t trustworthy.
We need to be careful about saying that someone must have a reason. The reality is that if someone can lie about something as fundamental as having a child, nothing they say is believable.
Depending on what job she does it could affect her employment, if she e.g. works in a position of trust then bing outed as a liar to that extent could be seen very unfavourably by an employer.