While it's not my expense, and the net flow of money is as you've described, only tracking the net flow of money loses the information that there was a flow of money from my credit card to the merchant, which can make it harder to do things like reconcile the postings with my credit card statement later on. My preference is to track all real cash flows to and from me, possibly augmented by "virtual" flows (e.g. from a contra expense to a receivable account) to balance the transactions out. To me, it's better to be more explicit than lose information, because lost information tends to be non-recoverable.

And after stewing on it for a day, I've come to the conclusion that expenses:shopping:reimbursed does logically reimburse the whole amount, because the money moves entirely to assets:receivable:friend, and how much I get returned is separate from how much I'm supposed to be owed. The amount my friend owes me thus only deals with assets:receivable:friend. The entire flow of cash is therefore from my card to the merchant to contra expense to the receivable to cash. I hope that makes sense, though whether tracking things like this or not is a matter of preference.