Just inform her that full time has higher status than part, however you will continue to try and work with their sched given you have the room. I always explain to parents that come for part-time care that it will be held on a contingent basis unless they chose to pay for full time spot. even then, I have let people go because they didnt use the space that someone else truly needed. Nothing wrong with being honest, and if shes really a friend she will understand by holding this spot she is also holding back your income.