Help me understand how there is even a remote possibility UA won't allow her to board. The routing is all on one ticket. Citizen of India (assumed), Origin USA, Destination India and Transit Japan+Singapore. Fairly straightforward Timatic search shown below.

1) UA absolutely needs to check the bag through especially since all carriers are *A. That's the whole point of an Alliance! Ask for a supervisor at UA or AI if you hear otherwise from either one.
2) In the very remote event that the bag is not checked through, someone in Changi should be able to transfer the bag for her as long as she goes to the Transit Desk. Wheelchair assistance should help take her where she needs to go and make everything much easier.
3) In the even more minuscule possibility that she needs to exit the transit area and go through immigration, it appears she will not*** need an SG visa for 96 hours, as long as she has a US visa (which obviously she would have) and all conditions are satisfied as noted below -- especially the "one-direction per journey" only restriction.

***Note the caveat regarding single-entry visas to the USA and other countries which allow passenger to qualify for VFTF. If in doubt, contact the Singapore consulate regarding application of VFTF to multiple entry visas when transiting SG and NOT on a direct flight to/from country that issued the visa. In general, the SG consulate is quite helpful and may even provide you written documentation.

In fact, Singapore even SPECIFICALLY allows passengers on two SEPARATE tickets to be able to check-in for the connecting flight in the Int'l transit area as long as there is not a Budget carrier involved.

People, please don't speculate - always check Timatic first.

National India (IN) /Embarkation USA (US)
Transit Japan (JP) /Transit Singapore (SG)
Destination India (IN)
ALSO CHECK DESTINATION INFORMATION BELOW

Singapore (SG)


TWOV (Transit Without Visa):
Visa required, except for Holders of onward tickets in transit

For details, click here.
Visa required, except for Nationals of India who can also

transit, for up to 96 hours without a visa under the Visa Free
Transit Facility (VFTF) on either the outbound or return leg
of the same journey (but not on both). Leaving the transit
area is allowed.
- ***Warning*** Holders of a valid single entry visa (or
multiple entry visa valid for at least one month) issued by

Australia, Canada, Germany, Japan, New Zealand,
Switzerland, USA or United Kingdom to nationals of India

may use the VFTF mentioned above. After using a single

entry visa and the VFTF was not used on the outbound leg of
the journey, the VFTF can only be used on the return leg
with a direct transit through Singapore from the country
that issued the visa to the country of residence (e.g.,
SYD-SIN-PEK).
- Transit passengers holding 2 separate tickets can check in

for the connecting flight in the international transit area,

except when For details, click here
Additional Information:

- Hotel accommodation is available within the transit area of

Singapore Airport (SIN).

Singapore (SG)

TWOV (Transit Without Visa):
- Transit passengers holding 2 separate tickets can check in

for the connecting flight in the international transit area,

except when arriving/departing on a budget carrier. Budget

carriers are: Air Asia (AK), Indonesia AirAsia (QZ), Thai
AirAsia (FD), Air India Express (IX), Cebu Pacific Air (5J),
Firefly (FY), Jetstar (JQ), Jetstar Asia (3K), Jetstar
Pacific (BL), Lion Air (JT), Scoot (TZ), Spring Airlines
(9C), Tigerair (TR) and VietJet Air (VJ).