As Ben rightly says, on the whole, starfish don't do well in captivity. Most starve to death due to being kept in aquariums that are too small or too immature to support their diet. They often develop wounds for no apparent reason and slowly disintergrate over a number of weeks, by which time hermits and shrimps often prey upon them.
Its not impossible to keep them but in an immature tank, and/or one smaller than 4-5ft, you will most likey require target feeding the creature. Again this can be problematic as other tank inhabitants like hermit crabs and shrimps, which move much faster than starfish, will steal the food intended for them. It also depends on the species of starfish too as some are easier to care for than others. Some are sand-sifters and will look for sustinance in the gravel. Others are grazers and will consume algae whereas some species are more opportunistic scavengers.
How big is your friends tank? What type of tank is it (fish only, fish only with live rock, or a reef tank)? What species of starfish is she interested in?