Dr_Rictus
First Post
Sure! Just, whatever you do, keep in mind that he's flat-out wrong. 
Seriously, it's been played extensively by some of the best, and while it's possible to shoot yourself in the foot with unwise character creation (either with or without multiclassing), multiclassing in and of itself does not imply any significant sacrifice in a character's viability. In fact, in some cases, it's a key component of an optimized character (the well-constructed fighter/rogue is a good example in my book).
Yoy might also consider that if he's interested in playing a fighter/sorcerer, as you cite in an example, that certain strategies might be needed at high level. Yes, this particular multiclass combination can be difficult -- that's not really a problem with multiclassing in general, in my view.
I find that the spellsword prestige class (Tome and Blood) is a good option for this character concept. If you don't have the book, encourage him to check it out, or look at it with him at your friendly local game store before you make a purchase decision (which you wouldn't necessarily need to make until he reached mid-level anyway). I don't want to make it sound like you need supplements to play a decent multiclass character in general (you don't), but running a 2-person party can be a challenge and having more resources at your disposal could be helpful.
One of the good things (and they are certainly not an unalloyed good) about prestige classes in general is that they provide a mechanism for encouraging multiclass combinations are interesting but that might otherwise be unattractive. In addition to the ones mentioned, there's the Rage Mage (from Dragon magazine originally -- was it reprinted in Masters of the Wild?). Good for barbarian/sorcerers.

Seriously, it's been played extensively by some of the best, and while it's possible to shoot yourself in the foot with unwise character creation (either with or without multiclassing), multiclassing in and of itself does not imply any significant sacrifice in a character's viability. In fact, in some cases, it's a key component of an optimized character (the well-constructed fighter/rogue is a good example in my book).
Yoy might also consider that if he's interested in playing a fighter/sorcerer, as you cite in an example, that certain strategies might be needed at high level. Yes, this particular multiclass combination can be difficult -- that's not really a problem with multiclassing in general, in my view.
I find that the spellsword prestige class (Tome and Blood) is a good option for this character concept. If you don't have the book, encourage him to check it out, or look at it with him at your friendly local game store before you make a purchase decision (which you wouldn't necessarily need to make until he reached mid-level anyway). I don't want to make it sound like you need supplements to play a decent multiclass character in general (you don't), but running a 2-person party can be a challenge and having more resources at your disposal could be helpful.
One of the good things (and they are certainly not an unalloyed good) about prestige classes in general is that they provide a mechanism for encouraging multiclass combinations are interesting but that might otherwise be unattractive. In addition to the ones mentioned, there's the Rage Mage (from Dragon magazine originally -- was it reprinted in Masters of the Wild?). Good for barbarian/sorcerers.