Worth browns are lower tension fluorocarbon and Martin 600s are higher tension fluorocarbon.
I suggest you friend first gets used to playing the one week old uke, then worries about the strings.
While getting used to the uke try a few things out to learn about the uke.
Tune it down to B or Bb for a session to check out how lower tension strings might work.
Tune it to C# for a session to see how some higher tension works.
Run along the C string at frets 3 5 7 10 12 with a tuner attached and see how varying fretting pressure sounds and intones.
Try some picks of various materials and see how they sound.
Think about the idea that a uke can have more than one sound depending on how you play it. If is loud, then the player has a range from soft to loud to play with. If its brash, the player has a range of brash to soft to play with. A player does not always have to sound exactly the same. If the string height is higher (not too high), you have a range of fretting pressures to play with. The person owns the uke, the uke does not own the person. You do not need to be changing strings every day to get different sounds out of the uke.