Friend has a 98 pathfinder that will not shift out of first gear. Mechanic says trans is shot to the tune of $2100. does she need to get second opinion or could it possibly be a sensor or solenoid in trans.

He's probably right, but you could try changing the fluid and filter first to see if it regains line pressure well enough to shift. Does it make any untowardly noises or does it just not shift?