You are both unwilling to help this young woman learn how to preserve food safely because you might upset her ?
That’s ridiculous. You are doing her a disservice.
it’s isn’t mean to guide your DSD into good housekeeping habits. It’s wise. And it’s your home.
She just isn’t getting the hang of shutting the door properly because neither of you have driven home how tricky the door can be and how careless she can be.
You don’t need to buy alarms and catches, you just need to be firm and take charge. Be confident about it.
Take her to the freezer. Say ….DSD do you mind if I show you one final time how to close this freezer door and also, how not to close it. It’s important you get it right now because I have had to throw a large amount of food away several times and so far the total cost of that is £xxx. We can’t afford to have that happen ever again simply becasue you haven’t learned how to close the freezer door properly. I know it’s tricky and that’s not your fault but the more food we have to replace the further away a new freezer becomes. So you now have to take full responsibility for doing it properly…like this, and never again neglecting to ensure the food remains safe and fully frozen by being careless and thoughtless instead of mindful of what I’m saying and of what you are doing.
Then let her try. Then show her how it bounces back if it’s slammed and have her practise a few times until she gets it right. Impress on her, in an exaggerated way, how important it is. You will be doing her a favour.
It might be an uncomfortable conversation for five minutes but it can easily be done without being mean. She has to toughen up a bit and realise her accountability. Help her out with that. She’s not a child. But actually even a child could be expected to navigate a tricky freezer door. It’s not ideal but it is what it is and the household has to adapt to deal with such problems.
She just needs a bit of firm guidance and a light telling off before it happens again.
It’s entirely normal to raise your expectations of her.
Her father sounds useless by the way. If he can’t guide her on this little problem, then heaven help her if she ever needs serious guidance eg learning to drive.