That's what would need to be considered. So far the process through the UK courts has been about whether Javid acted unlawfully under UK law. Did he de jure make her stateless? Several courts and hearings have said he didn't. Whether or not he did de facto could be another matter but that's outside the remit of the UK system. I'd expect that Begum's lawyers will be asking the ECHR to rule on whether or not she was de facto made stateless by Javid's decision, the legal implications of that if she was, and did it breach her human rights.