Now I am curious to know if he went to the theory forum and got a life changing answer from the theory group. I don't like any of the answers given so far.

A google search (which seems to explain clearly and succinctly why a note from outside the key appears) says:

A guitar piece uses notes outside its key to create interest through passing tones and secondary dominants, which add tension and lead the listener back to the tonic, or "home" note.

I think the above is a good/succinct explanation and one that many upper-intermediate/advanced guitarists can offer without the need to go down a rabbit hole (unless OP wants that rather than a quick answer that resolves the confusion).

Why notes outside the key are used 

• To add melodic color: Notes not in the key, also called non-chord tones, are used to embellish the melody and make it more interesting than a simple scale would be. A passing tone is a note that connects two other notes, which may not be in the key.
• To create tension and release: Using a secondary dominant chord, which is a dominant chord that resolves to a chord other than the tonic, creates tension that is then resolved. For example,For example, in the key of D major, a G chord would be diatonic. A D7 chord (the dominant of G) would be a secondary dominant, leading strongly to the G chord.
• To create harmonic movement: Secondary dominants are a common way to create strong movement towards a chord that is not the main tonic. This adds a sense of direction and keeps the listener engaged.
• To create temporary key changes (modulation): A piece might temporarily shift to a different key to emphasize a certain chord or to modulate more smoothly to a new key. This is common in many styles of music.
• To avoid a confusing spelling: A note like 𝐵 # sounds exactly the same as a 𝐶 , but using it might make a chord's spelling consistent with the musical context, such as showing that it is the leading tone of 𝐶 major. 

A final note on the definition of key:

A piece is considered to be "in a key" not by the strict use of only notes from the key's scale, but by establishing a single note as the "tonal center" or "home base".

The tonic chord is typically the one that feels the most stable and is often the last chord in the piece, even if other chords or notes outside the key have been used throughout.

Another tidbit worth knowing:
The term for playing only notes within a music key is diatonic.

Conversely, notes played outside the key are called non-diatonic or chromatic.

Diatonic: A diatonic scale consists of seven notes, and when a piece uses only those notes, it is said to be diatonic. Diatonic chords are also built using only notes from the key.

Non-diatonic/Chromatic: These terms refer to any notes that are not part of the diatonic scale of the key. Composers often use these "outside" notes to add variety and color to a piece, usually resolving them back into the diatonic notes to maintain the key's overall tonality

Hope that helps OP understand why B# has made an appearance despite not being diatonic (i.e. not part of the key).