It is if the ethos of collective responsibility still subsists, especially as she is charged with upholding law and order. On that basis, she is perfectly entitled and arguably required to comment on the breakdown of the rule of law within the seat of democracy of such a close ally, aided and abetted, as it was, by its Head of State.
Home Secretary, after PM and possibly Chancellor of the Exchequer, is the highest office in the land. Of course she can comment. To suggest it is beyond her remit to do so is absurd. She’s not a fucking diplomat.