Just say no. Tell her you haven't even used it yet. You should use your new bag first. Best friend or not.
One of my best friends asked if they could borrow my Spy. I told her she can borrow anything of mine except my handbags. I told her I wouldn't even let my mother borrow them. Not that shes ever asked but I wanted to get the point across.
First of all, I don't understand why she would even ask if she can borrow your brand new bag. Let's have some common sense. Just let her know that it's new and you want to be careful ad take care of it. She should understand. Hopefully she won't be consistent w/ asking to borrow it.
I'd loan out my husband before I'd loan out my bag........honestly!!
Tell she can borrow it later - but before you even have carried it!!! I can't believe she asked. Is it understood that if anything happened, she would fix/replace it?