The child is unlikely to be able to test directly in to a TAG magnet school because slots in them are assigned via a lottery system. Your friend could contact the TAG Office --
http://www1.pgcps.org/tag/ -- and see if there are any TAG magnet schools that are underenrolled for 5th grade and without a waiting list and then try to find housing in that school's boundaries.
The TAG office would also be able to help spell out what documentation of TAG identification is needed and whether or not a PGCPS screening is necessary.
I'd also say your friend shouldn't automatically write off PGCPS if there isn't a path into a magnet school. Look at the middle and high schools where her daughter would be zoned, as well as the elementary, and meet the principals and some staff and see if the neighborhood schools can meet her daughter's needs. Remember a lot of PGPCPS middle schools start at 6th grade, so she's looking at one year in elementary become middle school starts.