I think you are being emotionally blackmailed here. The girl who is organising the lunch clearly is sucking up to the one who is leaving, and is expecting everybody else to support her in doing it.
Dig your heels in and refuse to contribute.
You have already contributed to a leaving present for her - and after all, she was only there for 6 months.
State your case, say you have already contributed, and you don't wish to contribute again, and leave it at that.