The chances are that the guitarist started out either with a guitar package from a shop (including a guitar, amp, strap, lead, plectrum and tuner) or has been given a second-hand guitar. If it's the former, then most of the items in the kit are very basic and are chosen either for having a cheap price-point or a gimmick - maybe they've got green skulls on or some other pattern.
I've included some easy links for purchasing from Amazon.com and Amazon.co.uk at the bottom of the page. Don't want to read the rest, then go straight to the gifts in the US or the gifts in the UK.
Here's what you can get either guitarist:
A set of picks - buy a variety of picks. By the way, pick and plectrum are the same thing. Beginner guitarists and even intermediates are still finding their way around the different picks. Get a few thicknesses of each of the following: Dunlop Tortex, Dunlop Jazz II, Dunlop Jazz III. Maybe even get a copper pick. Don't bother with some famous brand on the pick, they're usually just regular picks with an image on so they can charge more.
Strap locks - if their guitar is worth less than $400/£200, then plastic locks (e.g. by Dunlop) are good enough. If it's worth more, then get some Schaller metal locks instead.
New Strings - They should be changing them on a regular basis. Play safe and buy what they use or take a chance or get them some great strings and see how they react after stringing up. As this is a rock guitar, get some Kerly Kues. As for gauge, go for 9s or 10s to be safe. If you know the string gauge, then go for them.
Guitar Stand - There are lots of stands around. Choose one that supports the guitar from the headstock at the top (i.e. the guitar hangs rather than rests). Better still look for one that locks the guitar in at the top. The Hercules guitar stands are good for this.
String Winder and Cutter - So much easier than winding with your fingers. A must for any guitarist.
Ear Plugs - No, seriously. Buy them some musicians ear-plugs. This is so important, it should be top of the list. If you're not sure, then try Elacin ER-20s.
Strap - Again, another personal preference. I like Leather or Leatherette with a padded strap for the shoulder and front of the guitar then tapering down to a thinner end for the back. I just go for plain black, but what's the guitarist interested in? Look around and you can find almost any design for a strap (skulls, football, alcohol brands, motorcycles, bandnames, roses, paisley and so on). Avoid elastic or stretchy straps unless you know they like them - it's a lot harder to play guitar when it's bouncing on a bungee rope. A warning: a lot of beginning rock guitarists have a habit of playing the guitar slung low, below waist height. This is great for throwing shapes and looking like a rock god/goddess, but not the best position for playing accurately. It works for some though. So if you're looking at straps, go for a strap that extends to over 1.3m if they're the low-slung type.
Cable or Lead - Buy a decent lead. You don't need to spend lots. Avoid the cheap coiled leads. Not all coiled leads are rubbish, but we can discount the majority. We're aiming for repairability, so get a lead with a non-moulded plugs. Something like a 15-20ft length will let the guitarist move around. I've had my last guitar lead for over 15 years and only retired it last year.
Part of an in-depth series looking at gift and present ideas for guitarists.