Sri Lankan Airways fly from Terminal 3 at Changi. There is no dedicated One World lounge there, so Sri Lankan (and I think Qatar) use the DNATA lounge. Having used this lounge recently, also en-route to Colombo, I would say it is not a place in which one would want to linger more than an hour or two.

If your daughter and her husband are arriving at Terminal 1, they may be able to use the very good Qantas or BA lounges, or if arriving at Terminal 4, the new and very fine Cathay lounge, however I don't know what the rules are regarding lounge use when one's departing flight is from another terminal (does anyone know if this is possible at Changi?) However even in the very best lounges I think 12 hours would be a bit much!

A lot depends on exactly when they will have their 12 hour transit; if it's during daylight hours, I would definitely recommend going into town for at least some of the time, if it's overnight, I would opt for a transit hotel (I think all lounges in Singapore close for at least a few hours during the night).