Hi -
Is he/she learning piano AND guitar from scratch, or have they done some music before?
Piano (or better still, keyboard) are much easier to learn and understand than guitar, which is physically difficult for months, until fingers toughen up and get used to where they should be.
As a primary TA I taught recorder and percussion from Yr2, and a bit of keyboard with Yr6, including MIDI, sequencing, and a bit of simple improvisation.
Keyboards today have hundreds of sounds and backings, for £100 upwards, and can be connected to computers for recording, multi tracking etc.
A teenager probably won't want to have to work TOO hard at making music, and so if it is just for fun and entertainment a keyboard will give quicker satisfaction.
If the aim is to STUDY music, and play the Classics, then a digital piano, or a more expensive keyboard with touch sensitivity might be needed.
I don't think there is any great shame in starting with music tutors aimed at younger children, at least to start with. John Thompson books are good to start on.
Or the Kenneth Baker series of books for adults or teens, which can be for keyboard, piano and organ, are well written and progress at a manageable rate. They also have some modern tunes, and 'Song Book' versions that have extra tunes in different styles, once some early stuff has been mastered.
Let me know what the person already knows about music, and what they hope to achieve and I'll try and give more appropriate information.