15 month old fighting at daycare

My daughter is 15 months old and stays with a friend and her 4 year old daughter while my husband and I are at work. She’s our only child, so she’s generally used to a lot of attention at home, and she is very very active.

She’s been staying with my friend since she was about 5 months old, it’s been absolutely wonderful and the girls have gotten along really great up until recently. My friend’s daughter is generally very sweet and helpful and will tell my daughter no or stop her when she’s doing something she shouldn’t be doing, but lately my daughter has been biting/hitting/scratching/pinching my friend’s daughter when she tells her no or just when she’s doing something my daughter doesn’t like. Since she’s an only child, we really don’t run into this issue at home. She will definitely throw fits and try to bite/scratch us every once in a while, but it’s certainly not common and she stops as soon as we tell her no.

Is she still too young to understand discipline?? My friend usually tries to separate them when they aren’t playing nicely, but my daughter’s behavior doesn’t seem to be improving. Looking for any tips or advice we can implement at home or at daycare to hopefully help.

My friend’s daughter used to be so excited when we got there in the morning and she just isn’t so happy about it anymore, so any advice is greatly appreciated and welcome.