Oh yes, these other posts have jogged my memory, I forgot about the money stolen from my bank account with my card. I couldn't get out to the cash machine and it was agreed that carers would do it for me. Strangely I've stopped that part of the carer job now, after being stung a few too many times. Now though I have to have large amounts of cash in the house as Im mostly bed bound, and carers need petty cash for shopping etc. I get a choice of hiding my money securely but in a place that I can't access without pain and risk of injury, or having them money where I can get to it, but so can thieves... But that's the thing about being disabled (or elderly), you are put in positions where it's easy for people to exploit you.
Btw OP, I hadn't read all the replies when I went on my massive rant, so didn't realise there was a need to caveat my response.
But just to make it clear, I don't think you are a bad friend or anything & I'm sorry that anyone implied that 
I was purely focused on what your friend had done, and how her excuse just doesn't wash.
There is no valid excuse that starts 'I took advantage of someone more vulnerable than myself and had the opportunity to do so as I'm in a position of trust. But it's ok, as I'm very vulnerable too, and someone is behaving terribly to me, so I decided to behave terribly to someone even more vulnerable than myself'
Having said that I'd still help her get out of her relationship if she actually wants to - not in a 'lending money for some tenuously related thing that at some point may ladder up to an escape plan' way though! I'd help her with info, advice and some practical support (like taking her to housing office).
But I am skeptical she actually wants that kind of help, as it all seems aimed at keeping her in the situation vs getting out - he takes her money she needs for petrol so rather than moving out she steals money from someone who can't protect themselves... All very dubious tbh.