Depends on what your friend is interested in, what you are interested in and whether he will be spending more than one day in Singapore so will visit other places on his own.
If he only has one day too, I suggest wandering around the markets and temples of Chinatown, Little India etc, perhaps combined with half a day in the Botanical Gardens or Jurong BirdPark - both worth a visit. I thought that Kong Meng San Phor Kark See Monastery was worth going to as well, but it's quite a trek to get there and there's not much else around.
I hope you know some good restaurants to take him to for dinner but, as a suggestion, having a Singapore Sling (or two) at Raffles' Long Bar is generally a 'must' for all tourists and could be followed by dinner at one of the restaurants in Chjimes or along Boat or Clarke Quay, unless you have transport to take him out to the beach area for a black pepper crab.
Banana Leaf Apolo in Race Course Road served very very good food, if you happen to be in the Little India area around lunch time.
I worked in Singapore for two months recently and had plenty of time to go sight-seeing at weekends.