Depends on his/her background and age.
Fresh water tropical are generally the easiest to deal with. They have a greater variety, lower cost. Mollies, platies, sword tails are some of the most tolerant of these for most people.
That said they should learn about the nitrogen cycle and get a complete test kit: pH, Ammonia, nitrites, nirates. If you have a good LFS have them run a complete set of tests on your tap water. If you have hard alkaline water Africans may be easier to keep, softer well or filtered water south American species. The less you have to screw with the water the easier it becomes. Also no matter how hardy a fish is lack of water changes will kill it.