While that sugar in itself, probably isn't immediately dangerous - it may be a sign that there is a significant illness brewing up, or that the medications she is on are not suiting her.
A sugar that high will certainly be causing her inconvenient and unpleasant symptoms - thirst, need to go to the bathroom loads, blurred vision, etc. She should be seen by a doctor soon who will be able to advise on what to do.
While I don't want to speculate on why this has happened - a common cause is high dose steroid medication, which can cause temporary diabetes while they are being taken. If this is the cause, then some diabetes medication may be needed until the steroids are stopped.