Don't really think you should be getting much sympathy to be honest. You don't have a receipt so they're within their rights to say no refund. Old mate should have probably said it more tactfully but that doesn't detract from the point that you were technically in the wrong.
You complain about non-existent customer service, but that's the point of Aldi and part of their USP. You look at the number of staff they have per store versus Coles or Woolies, and you'll see how they can sell the products they do at the prices they do. A lack of customer service relative to those others stores is a result.
How much do you also value your time? You've had another trip to the store, probably wasted about 30 minutes of your time. Emailed head office, and had a call, which is probably another 15 minutes wasted time, and now posting on here and ringing up your mates to try and boycott. It's honestly not worth it for $25 especially as you lost the receipt and are technically in the wrong.