The NPC WAS the one that pretty much hired the party for the module task at hand in the prequel sequence I made leading up to the module's actual starting point. He doesn't exist in the original module but was added in. That said, if the party goes by the module, there's no need to fight against him. But if there is a combat sequence involved, he's not gonna let himself get killed by the orcs attacking the party. Technically he's not even apart of the party during the main quests of the module.*Was this 10th-level NPC put in the party 1) by your-as-DM choice, or 2) by player/PC request or recruiting, or 3) by the module itself?
IME if it's 1) things rarely if ever work out well, and if it does for you this time you're probably rather lucky; if it's 2) the players kind of brought it on themselves; and 3) sometimes can't be avoided if running a module as written (e.g. in the original ToEE the party has the opportunity of rescuing (or waking up?) a Paladin who is likely to be at least double the party's average level).
I try to make party-member NPCs at or slightly below the average party level when they come in, and they progress from there just like a PC would. If a party NPC isn't needed any more, or - as happened in an adventure I ran not too long ago - the part accumulates a bunch of them, I'm quick to pull the trigger on retiring them. (once back in town the 'bunch' all left to form their own all-NPC adventuring party, which gives me some potentially handy plot options to work with later)
He was referring to a character that is a senile old man in a nursing home.
Roughly-equal contribution to combat isn't really a concern of mine provided those whose contribution to combat is lesser can make up for it in other ways. In the Rogue's case, this make-up realm is - or should be - exploration.
Ranged sneak attack bugged the hell out of me in 3e when I met it there, and still does.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.