Broken spokes - When to replace all of them ?

[Andy Dingley]

I'm resurrecting a wreck for a friend (so the budget is minimal). It's a mid-90's Diamondback MTB,
which is going to be used for commuting and some very easy off-road. The rebuild has gone as far as
a complete strip-down and frame repaint.

While trying to get some tension into the flaccid spokes, a rear drive-side sheared off just inside
the nipple. Looking under the microscope, there appears to be a small corrosion pit. Should I just
replace this spoke and continue, or would now be a good time to rebuild it with all new spokes ?

When is it an indication that all spokes are likely to be suspect ? Generally the bike is in good
condition and little used.

Thanks for any advice.