That would be the case I suppose if John's entire contribution was poor. I was thinking perhaps there might be some part of John's contribution that was actuall

That would be the case I suppose if John's entire contribution was poor. I was thinking perhaps there might be some part of John's contribution that was actually quality and to say "hey, you know you're doing good work, it's just this part that isn't working well". Obviously we're dealing in hypotheticals here and I guess if it is the case that none of John's code works well then your approach is probably the only decent approach. If there is some code of John's that does work well than I find a lot of times it's easier to give a little compliment prior to your criticism just so they know that yes, we do appreciate your work and contributions, but in order to be more effective here is what you have to do.