Originally Posted by
angetenar
If it was originally booked as two separate reservations, UA didn't sell anyone an impossible connection.
For OP, HND-NRT transfers are not currently possible. How important is the trip to you? I see several options. UA is expanding HND flights so you can see if you can get moved to those, which should be allowed because you were affected by a schedule change. Alternatively, you can see if you can fly SIN-MAN-IAH on SQ.
Ah, I did not see they had two separate tickets. In this case your advice about HND is good, or they can try to move to a NH operated codeshare HND-LAX/SFO then connect to IAH on UA if space is open.