In some circumstances, it may even be violating your termination agreement with the former one.
Often, if made redundant by a company, you cannot return to work for them (as employee or contractor) for a year, otherwise you have to pay back your redundancy pay.
That leads to a potentially hilarious conversation about rates.
"I'll do it, my rates are 100 per hour plus 10K if you want me to start before June next year"
If you don't want to tell them to F Off for this reason, you can contact their line manager, HR or customer service department and ask them to do it for you.
The kind of person to lay into you like this is not going to give you a glowing reference whatever you do.
If someone from my old employer calls and asks me politely for a bit of help with something I know well, I'd help.
If they call to yell at me about how I left behind a fragile system or how I left them in the lurch by quitting, the correct response is to tell them to get fucked, if they then ask for help after that, the correct response is to tell them to go fuck themselves.
Normalising that kind of abusive behaviour is not going to improve the lives of the co-workers you left behind.
e.g. you would ask, "Can you swap us 5 nuggets for a Lady?" If you wanted to go and get something from a vending machine, but only had notes.
Even more so than other terms, I think this is being lost to cashlessness.