Indigo has one big bank of checkin counters in CCU. You can't miss it. 'Fast Forward' checkin is right at the tip of bank C (see below)


On the way back, she will arrive at the lower level. CCU Intl and domestic is in one terminal since about 10+ years. Just find the transfer/check sign and take an elevator/lift up to the departures level and look for the SQ counters in the checkin hall (Bank D or E). Maximum time required: 5-10 minutes. Usually the SQ counters will open 4 hours before departure. She can use the waiting facilities that AAI operates below the departures level to wait. (they are called 'retiring rooms': https://www.aai.aero/en/airports/pas...a/Conveniences) There is no landside waiting lounge. Once airside there is only one lounge that you can access with PP. The TFS lounge in both domestic and international is pretty decent.