Nah, he profited. 2p in every pound - might not sound like much, but if you've got multiple people putting their shopping through, that's easily hundreds in a very short time; DP's got a standard card with £90 sitting in it right now, just from day to day, low value shopping (no cat food or litter, toiletries, cleaning products or expensive items, just vegetables, milk, meat and coffee).
It's not news to anybody in staff that this is theft - exMIL was very clear about it even 20 years ago, just like when I was a Saturday tills kid in the 80s, you were told it was a sacking offence to keep any promotional stamps if customers didn't collect them (and before that when Green Shield stamps were a thing).
ETA: It's certainly not secret - it's even available just by a quick internet search
https://colleagues.coop.co.uk/colleague-membership
...colleague membership, and the discounts and benefits it gives you, are for your use only and should only be used on things for your own purpose. You mustn’t give, lend or sell your card to anyone else to use,
<snip>
If we suspect that you’ve misused your colleague membership card or that your partner card has been misused, we’ll take this seriously and investigate in line with the Disciplinary Policy. It could result in disciplinary action being taken, including summary dismissal, and your membership card being taken away...