Guitarists can never have too many guitar strings! Some people think you only need to change your strings when one breaks, when in actual fact, pro guitarists will change their strings every couple of weeks, or even before every show! It’s a "tonal" thing. New strings sound sweeter, feel nicer to player, stay in tune better, and simply bring your guitar to life.
The only thing you may need to get right, though, is the string gauge and type. Every guitar is specifically set up for a certain gauge of string (the thickness), and if this is changed without a proper setup then it can affect the neck and, ultimately, the playability of the guitar. Fortunately though, the majority of electric guitars come with 9-42 or 10-46 strings as standard. You'll also need to make sure you get electric strings if they have an electric guitar, and acoustic strings if they have an acoustic. It may seem obvious, but buying the correct strings for the guitar is crucial for it to sound and perform properly!
Guitarists generally have a favourite brand and gauge of string, so you may need to sneakily find this information from the person you’re buying a gift for!