You might want to tell her to go online and take a look at other fish just in case. Once she has a damsel in her tank it will be very hard to get out if she wants other fish.
You could also show her some online forums. That way she can look at other people's tanks and get ideas.
Once she knows exactly what she wants (inverts, fish, etc..) then its time for real planning.
They are so aggressive that they would kill each other off until there is only one in that tank size. My friend had four in her 55G. After two months there was only one.
Edit: Thats why I suggested she look into another fish if she wants more than one.