So imagine your partner bought a lottery ticket, wasn't a regular gambler, did this once in a blue moon, maybe £5 once a year or so. They win 12k. Then spend it all on more gambling (but no more than this) but lose it all. They stop. Haven't gambled again since. Genuinely haven't, otherwise very financially responsible. Aibu to wish they hadn't blown the 12k? Or, was it their money to do with as they please and no issue because they started with a balance of -£5 and ended with a balance of -£5. There is definitely no ongoing gambling issue.
Yabu - you're being unreasonable, none of your business, no harm done.
Yanbu - yes, they blew thousands on nothing.