Your friend needs to be clear with everyone. Although it's been more flexible in the past, she will need to have 'same rules for all'.
A parent handbook helps. I've written two of them.
Yes, she should have regular, routine hours of care that she is willing to provide, and to make this clear. For example, if she ends care at six, she should expect to enforce this with measures to hold families accountable for picking up on time. etc. Families need to know if there is a late pick-up fee, if there is a one-time grace period for this, etc. She need to make some choices in this regard. As someone who has run a preschool from my home, I made it very clear when people were welcome to be on the property.
Payment should be uniform for all parents, in my opinion. If it's monthly, she might offer to take payments consistently on the 1st or the 15th of the month. There should be a short grace period (I offered three days) and then a reasonable per-day late fee. She should also be clear about when she will NOT provide care due to lack of payment. I personally think that six days out from the agreed-upon payment date is time to say 'please don't bring your child until I'm paid in full'. She needs to make this clear and then stick with it. There's nothing worse than getting a reputation as being too flexible on payments. People stop taking you seriously.
Are there fees or deposits she needs from parents? Also, how much notice does she want from them if they are going to move their child to another care provider. I recommend one payment cycle at least.
Create the daily routine. This should be a schedule which includes meals/snacks, times for some specific activities if possible (storytime, messy art, music and movement, outdoors play, free play), and naps. She should be clear about when meals are so parents dropping off later aren't expecting her to serve a meal she cleared up a half-hour ago. Routine is also important for the children. She should know what her 'day with the kids' looks like and be able to convey that.
Expectations: what does she need parents to bring, either to leave for her to store or for them to bring on a daily basis? How many diapers/wipes? Changes of clothing? What sort of outdoor clothing will they need? Will she be providing meals and snacks, or should parents be packing a nutritious lunch?
She should also have an illness exclusion policy which covers in detail when kids should be kept home.
In the care situation she's in, she might consider creating some sort of policy regarding exclusion due to aggressive behavior, and to be specific about when a child needs to go home. For example, a toddler biting is more appropriate than a six year old biting or aggressively hitting and should be treated differently.
I could go on. My best recommendation is for your friend to visit a couple local daycares, take the tours, and ask for a parent handbook if possible. This will give her some idea of what she might want to include.
Oh, and she might consider purchasing an insurance policy, or creating herself as an LLC as a firewall to protect her household interests. I did both to make sure we didn't lose the house, just in case a child was accidentally hurt.