Since it is a FIRST scheduled test event, my understanding is she will be rescheduled for the test at a later date.
Beyond that, the question is significantly more complicated than whether difficulty returning to Canada given her reasons for being abroad is an "acceptable" reason for rescheduling or failing to attend. There is NO certain answer.
In particular, how IRCC responds can depend on additional circumstances. For example, did the applicant previously notify IRCC of dates the applicant was going to be abroad? Has the applicant properly notified IRCC of any changes in material circumstances, including current residential address (the place where the applicant is actually residing)? Even the nature and strength of the applicant's ties to Canada can be a factor.
In the past there was an operational bulletin which specifically stated that living abroad was NOT an acceptable reason for failing to attend a scheduled event. This does not appear to be included in the current guidelines, as specified in the applicable PDIs (Program Delivery Instructions), which tend to be more general if not vague. Moreover, even if it did, technically that applied to excuses for a failure to appear and not requests to reschedule. And, again, as I understand it applicants are now given a second chance to attend the test event regardless the reason. Nonetheless, the previous policy and practice is a clue about how Canada approaches such questions. But that said, in contrast, even during the more strict approach CIC employed during the Harper years, going abroad for an obviously temporary purpose related to a particular graduate program was often given more of a pass than other reasons for being abroad.
And as
@bluffmaster88 has suggested, there are other factors to consider beyond whether this will result in the application being deemed abandoned and terminated (again, as I understand things, for first scheduled test this does not happen). But here too, how it goes can be influenced by a wide range of other particular circumstances, not the least of which is the underlying strength of the case itself, including the nature and extent of ties in Canada.
Just asking the question suggests the applicant will weigh competing priorities in deciding whether to seek rescheduling or make the sacrifices involved in making the trip to attend the test. If not, if the applicant has already determined attending the test as scheduled is NOT possible, not going to happen, given her situation, the thing to do is make the request to reschedule regardless the impact on the application. And she will find out, in her particular circumstances, how that will impact the process. The thing to be aware of is that this can significantly elevate the amount of scrutiny given her application and will quite likely result in a much longer timeline overall.
