There's no service for that as she will have to directly check in her own luggage. If there is no transfer of luggage, the ONLY way is to go out landside. Which means pass immigration, visit baggage claim, then enter the main airport area and go upstairs to Departure Check In for United.
It's correct she doesn't need a visa to transit and stay airside, but because of bags - she'll have to go landside if there's no interline agreement.