Today I will answer this in the form of a movie.
The Good - Candle Wax is has extremely low conductivity so the probability of you damaging essential electronic components is very low to no chance.
The Bad - Make absolutely sure you are getting the right part before you order it. The last thing you want is to buy the part only to find it does not fit. I recommend taking it apart and making sure everything matches before ordering.
The Ugly - Your tech savvy friend better be pretty good because this is a difficult system to get in and out of. Additionally if candle wax did get onto the motherboard (which I doubt it did), then he should not use any metal tools, excessive force, or a lighter to clean it off.
The End - There is an excellent guide on how to get to the upper case provided by the iFixIt staff and I have the link [guide|15404|right here].