My husband thinks it may be the vacuum. The heater core in your daughter's car is partially plugged or restricted nearly stopping the flow of coolant through it. Flushing it out or replacing it should correct her problem. Heater will not get hot. Temperature gauge is normal.
Air is just barely warm.
Have replaced thermostat, vacuum control switch in dash and heater core. Have bypassed the valve at the firewall. Water hose to heater is hot to the touch. Return water feels cooler to the touch. Sounds like you are having trouble with the doors in the heater duct not opening properly.