Actually, looking at the law it might be legal under Article 8 Paragraph 3 b:
"3 . Notwithstanding the provisions of paragraph 1 of this Article, a Contract- ing State may retain the right to deprive a person of his nationality, if at the time of signature, ratification or accession it specifies its retention of such right on one or more of the following grounds, being grounds existing in its national law at that time:"
"(b) that the person has taken an oath, or made a formal declaration, of alle- giance to another State, or given definite evidence of his determination to repudiate his allegiance to the Contracting State."
One could argue that when she joined ISIS she pledged allegiance to the "Islamic State in Syria".
Or even based on Article 8 paragraph 3 a II:
"(ii) has conducted himself in a manner seriously prejudicial to the vital interests of the State;"
One could argue that her joining a terrorist force that is threatening western soicety is a manner prejudicial to the interests of the state.
Source for the law:
https://www.unhcr.org/ibelong/wp-co...ion-on-the-reduction-of-Statelessness_ENG.pdf