I really got into music during the whole nu-metal garbage phase of the late 90s, and I had a real objection to any kind of guitar solos, keyboard parts, etc. Broadening my listening I found other bands/artists who played guitar solos without sounding wanky, and now I rarely listen to bands that don't let their individual musicians have their moment in the spotlight. However, I'm still repulsed by garbage shredding type guitar playing, or over-the-top, cheesy keyboard parts. It's just not good music. It's like, 'wow, okay, you can play really fast.. good for you.. can you actually play something that sounds _good_?'.
In relation to the original post, I used to have that same opinion about 'guitar-based' music. I used to think anything with solos was 'teh gay!10!'. But since then, I've learnt better, and I love me a good guitar solo. But it's not from a standpoint of being awestruck by the technical abillity of the musician; its about the musician using that ability to create gorgeous melodies. I don't understand people who like shredding guitars; seriously, that stuff is gayer than anal sex. It's unlistenable. But a great guitar player who can use that skill to create beautiful soaring melodies is just brilliant.
So, again in relation to the original post, your friends will only broaden their horizons when they decide to do so. Tool fans in my experience are very closed-minded about other bands and other types of music. Tool fans love to rant about how deep and spiritual Tool's lyrics are, when in fact plenty of the them are just stolen from other sources (comedian Bill Hicks being the obvious one). Pointing out that Tool's musicianship isn't _that_ good only seems to elicit responses in the vain of 'oh, well you just don't _get_ it' (like that actually means something; no, I get it, I just don't think it's particularly good). These people will only open their minds to better music if they decide to do so; thrusting different/better styles of music upon them only seems to strengthen their Tool love. So let those people stew in their mediocrity, and enjoy the adventurous sounds of bands like Opeth.