As a resident of Brisbane and a frequent traveller to Sydney (and user of hotels), I have to agree wholeheartedly with JohnPhelanAustralia’s choices and his dislike for Sydney Hilton.
Whilst the IC can be reasonably priced and has stunning views (and proximity to trains, Circular Quay, Opera House and ferrries), it is on the CBD fringe. Having said that, I’ve never stayed there.
The Westin is the most centrally located (middle of George Street). I’ve stayed there numerous times due to proximity to my clients (on George Street). An easy walk to anywhere and nice conference facilities. Walk down to Wynyard (train station) is about 400 metres; or up to Town Hall.
Sheraton on the Park is on the upper fringe of the CBD and directly opposite St James (train station). It has great views overlooking Hyde Park and good conference facilities. 3 blocks down to central CBD, a little further to Circular Quay (I’d catch the train).
If happy to look to Darling Harbour, the Four Points Darling Harbour I would rate alongside Crowne Plaza Darling Harbour. The Four Points is a little older and rooms aren’t to the same quality as Crowne Plaza.
Presumably your friends’ clients are based in Sydney. A lot may depend on price and conference facilities, rather than the room.