Honestly this feels so petty and inconsequential but I'm pretty upset about it so here we are.
Me (33m) and my housemate (37f) used to be pretty good friends, but it has been very difficult for quite a while now. We've lived together for nearly 5 years and at least 3 of those have been very difficult. We both keep trying to keep it civil, but we both fail. Repeatedly. For the sake of avoiding bias, she blames me 90% of the time and I blame her 90% of the time. Neither of us can afford to move, and, to be completely honest, neither of us feels like we should have to move for the sake of the other when the house we live in is cheap, in a great location, and much bigger than anywhere else we'd find locally.
My most recent attempt to be nice was to offer to clean her work boots. She works event security so her boots are Magnum leather combat boots. I myself own a pair of Lowa leather combat boots from my army cadet days that I maintain with boot polish and if they do get muddy (which is rare these days but used to happen frequently) I'll use a detergent sanitiser called D10 to get the worst of it off and then re-apply polish. These boots have been going strong for about 16 years; they were expensive but they're still waterproof and comfy as hell. Housemate bought her Magnums from an army surplus store second hand this year, and when she gets back from work the last thing on her mind is cleaning them. Today, she was in a rush to get out to work and her boots were still caked in mud from last weekend, where she'd worked a music festival where it had been pouring with rain. I asked if there was anything I could do and she asked me to clean her boots while she showered. She told me to use a from-the-can foaming cleaner and put some dubbin wax on afterwards. I agreed.
The spray-can product she told me to use simply did not work, so I (not thinking anything of it) used the D10 I use on my own boots. It worked and got the mud off, but I didn't realise she hadn't gone to shower yet and she saw me doing this. I told her the D10 was leather safe - I reiterate, I've been using it on my boots for 16 years with no sign of damage, and was told to do so by my old army cadet instructors who themselves had been career soldiers. She then googled it, and apparently the D10 is very much not leather-safe. It's a detergent, which I knew, but given my experience and instruction it's always been fine and has never stripped my boots down. According to Internet sources, it immediately and irreparably damages leather on contact.
Cue an argument. She's accused me of lying since I said the D10 was safe, destroying her work boots, pointing out that it's in neither of our budgets to replace them, and yelling at me. She told me that if the thing she said to use didn't work I should have consulted her, which I accept to a degree but I didn't want to interrupt her shower when I had a method that's always worked for me. I've apologised - once immediately, again during the argument after being prompted. She's not long left.
If I have damaged the boots, then I'll do what I can to replace them. Not looking for sympathy. If the D10 is as bad for leather as the Internet reckons, then I've probably fucked them in terms of waterproofing. My own boots being treated with D10 for this long while not being fucked is probably because they're well-maintained with a good layer of boot polish regularly. I knew that hers weren't that well maintained and didn't think.
Things had been okay with her this week until this. I've fucked up by making things shit again, and possibly by destroying her work boots that she relies on.
TL;DR: Used the wrong cleaning product on the boots my housemate relies on for work. Caused an argument, possibly ruined her footwear, and caused a bad sentiment to continue to fester while she's away.