I think this can not be done today. I use a category called "refund" in both expenditure and income and use it to buy some custom which later I returned the amount by payment. Since these are not costs or actual income, ideally to exclude both budget and various reports.
You should be refunding expenses in a different way. Just find the original expense transaction in the Account register, tap on it to expand it and then select Options -> Refund.
It will look like you're adding a new transaction. You can modify the target Account, Categories (if applicable), amount, date, etc. If you were refunded in full, just leave the amount as it is. Otherwise, modify the amount there.
Save the refund and MoneyWiz will take it into account when calculating your budget allowance as well as when generating reports :)
We think you are complicating things for yourself. When you make a purchase for your friend, you don't need to create any categories and you don't have to name anything as refund. When doing an expense, for example - shoes for your friend as you said, you go to MoneyWiz and create a transaction with Category "Shoes". Then your friend gives you back the money for these shoes. What you need to do is to find the original expense with category "Shoes" and click on Options, and then choose Refund. And the rest of it, as my colleague Paweł described above. If you do this this way, the amount for this purchase will be excluded from the report, just as you wanted.