The problem is that it is not possible to say with certainty that she will be able to proceed directly to departures level, i.e. bypass transfer security checks. In principle, flights arriving from the US should have that option as arrivals from the US are considered "clean" but there are several reports that there was no TK staff on the aerobridge to check onward boarding pass, i.e. also transfer pax needed to follow the signs for and proceed through transfer security checks. It will be impossible to tell in advance whether TK staff will be present in time for transfer pax to proceed directly to departures hall/level.

1½ hour at IST is plenty of time/OK. Incoming flight can be 30-45min late without jeopardizing the transfer.