Yeah a star fish wouldnt survive long in that size tank.
A "nemo" setup isnt that difficult all you need is as follows.
The 20 long tank
10-15 kg's of live rock
2 or 3 powerheads to circulate the volume of the water atleast 20 times.
No additional filter is necessary or advisable
A small layer of sand on the bottom i.e 1/2 inch
Lighting depends on what corals your friend would like to keep, for the fish and live rock a normal bulb would be ok, preferably 2 bulbs.
If she wants to keep corals then look for the t5 or t5 compacts which for the same length are 2or 3 times brighter.
She will need to use ro water or algae will take over the tank depending upon the tap water, you can buy ro at a lfs fairly cheaply.
For stocking a pair of tank bred clowns and possibly one other small fish, no more and a clean up crew of crabs and snails.
If you look at nano-reefs.com theres loads of info there on small reef tanks, aslong as you take your time, test the water regularly and dont overstock then they arent that difficult imo