My new subletter l moved in on May 1st. I JUST found out I’m going to need to move in October. When should I tell her?

I have a two bedroom apartment and am the sole tenant on the lease. I have always had a roommate in the second room (they aren’t technically on the lease, but it’s all squared away with the landlord). My lease is month-to-month, which this roommate is aware of. She just moved in on May 1st, and I just found out two days ago that I need to move by October 1st for familial reasons. I could offer to transfer the lease to her, but I need to sell most of the furniture. Should I tell her immediately, even though she’s still settling in? Or should I wait until she’s not in the panic of unpacking? Sorry if this is a dumb question, I genuinely don’t know the right thing to do. She’s a stranger, but she’s sweet and I don’t want to leave her in a lurch.