We just let our 15 and 17 year olds (rising senior and rising sophomore) stay home alone for the night for the first time. We were about 45 minutes away and we’re gone from 5pm until 10 am. Had a neighbor they could contact in case of emergency. Also, good (well behaved, nerdy, Rule follower) kids. We knew with confidence there would be no parties, etc. It was was fine. And I would have no problem extending then to 24 hours.
The summer they were 16 and 14, we let them stay alone until 2 or 3 am when we went to concerts in Maryland (NOVA based). The summers before that, from 14 & 12 upward, as late as 11-12 for a Wolftrap concerts, movies, etc.
This progression has been very much in line with what their friends parents have been doing.
I felt better because they were together. In fact, we moved our overnight so they were both home from camp.
If you are in Fairfax County, I would be hesitant to do it at 15 because the Fairfax County guidelines are you can’t stay alone overnight until 16. They aren’t binding, but if something goes wrong, I’d be concerned about CPS getting involved.
https://www.fairfaxcounty.gov/familyservices/children-youth/child-supervision-guidelines
Otherwise, ask: how late at night has she been alone before, how often, how did she react, is she an anxious kid, is she trustworthy, how close by is grandma and can she come at 2 am in case of a problem? I’d this a reasonable step upwards in independence or too big? Is she ready?
Asking for a friends implies she might be a little scared about being alone. Under no circumstances would I let someone’s else’s kid stay with my 15 year old.
As PP said, grandma could stay at your house for a night or two.