Can I ask why doesn’t she want NHS to know?
It does kind of sound like an excuse not to get the help she needs…
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You can make your decision however you have got to have boundaries, if she’s stealing from you then you need to consider if you can trust her at your place – I’m guessing not – I totally get you’ll feel somewhat responsible for her in the way she has nowhere to go etc (doesn’t she have family?)
If she uses/ steals for her without facing consequences then you are enabling and what reason does she have to get clean? She’s getting the best of both worlds/ with regards to money for food is a massive no no… regardless of the amount…
Get some boundaries in place and stick to them then go from there… if she excuses to get help the. You’ve got to think of yourself…
Good luck to you and your partner I hope she gets the help she needs