Just a theory.
Friend longed for the airwrap and was impressed with it.
She decided to keep it for a bit longer maybe over Christmas/NY, afterall she'd just got married and everyone ought to be nice to her and the OP wasn't chasing her as such.
That is initially why she strung out the return.
She then got used to having it and started to feel it was hers - why shouldn't it be hers. What does the OP need it back for, she can afford it etc...
She's got form for being a CF and delaying payments etc until people give up and decide its not worth it, so she felt with a bit of delay etc that the OP being generous would agree to write it off as a wedding present as this had worked with smaller bills in the past.
She made up a story to see how far she'd get but then realised from the OP's reaction that she'd dug herself into a hole,
She was aware of the Boots offer but wouldn't go for that because by then she was annoyed that the OP kept asking for her property or the money back and this was "taking the shine off her wedding" ( entirely her own fault)
there's the breakage theory but It feels like a difficult item to "break" or if it was she could google repair and find out that its not that difficult. She hasn't done that either.
I don't think she would have sold it. That takes organisation and effort and she really really wanted to borrow it in the first place, probably more than the cash.
She previously said she would pay for replacement on credit card but that would get her into debt. Credit card spending is debt. It's really a pre agreed loan. £500 is a lot to lose and replace but it can be paid back at her own pace into the credit card. She said she was willing to repay OP for the cheap version over several months, so CC was her best option really, to resolve the matter without "taking the shine" off her own wedding. But she doesn't really want to do that.
She thinks the OP will be swayed by this.
She's now Playing the Ostrich, hoping if she doesn't reply OP will give up and go away, ditch the friendship as the Friend no longer wants the hassle of being confronted with her CFery and she's seen that OP is no longer being the useful pushover.
Now she's ghosting you OP, I think you should initiate the court proceeding. You will probably get a resolution way before getting to court. People around her will be so fed up of this by now, since it is probably all in aid of covering up her dishonesty that they will tell her to just settle and move on.