Does the money paid by the plaintiff, which she did not own or because she forgot her card, constitute ill-gotten gains consumed wrongfully? And what should she do?

For each individual in a group to contribute their food to share in eating is a legitimate and commendable act, and it is a form of cooperation and tolerance. If it is customary among a group to distribute the cost of food among themselves, such that one who eats without paying is looked down upon, then this customary condition must be adhered to, based on the Prophet's (PBUH) saying: "Muslims are bound by their conditions." So, whoever eats without paying, claiming straitened circumstances, has eaten the property of others without their permission, and this is not permissible, based on his (PBUH) saying: "No one should milk the livestock of another without his permission." If you think they will not forgive your participation without paying, and you have money, then you must seek their absolution. If you fear scandal or spoiling the relationship, you can invite them to a meal at your own expense to compensate for what you ate, or give them a gift equivalent to their food. Lying and bearing false witness are not permissible, and one must repent from them. You can excuse yourself from participating if it harms your finances or when you do not have what to pay, but participating and hiding your money from them is blameworthy.