I wonder if this is possible...

Since their vacation would be in Hawaii (and if they don't mind spending extra couple of days in Singapore of BKK), I would suggest this...

for 90K UA/*A saver award,

IAD-EWR-SIN (SQ) or IAD-JFK-BKK (TG) for the outbound,

Stay in SIN or BKK for couple of days

SIN/BKK-NRT-HNL (stop over for several days) - ORD/DEN/SFO - IAD for the return on UA exclusively.

All in C and F (for HNL-IAD portion).

If this works out, then you get to spend sometime in tropical asia as well as wonderful Hawaii. Yes, this would also require you to get HNL-LIH/KOA tickets, but those shouldn't be too expensive. I think this would be best bang for mile in my opinion that would accomodate the OP's original choice of destination. What do you guys think?