Let's step through this line by line, looking for a spot where the friend could have made a mistake.
So, where could the friend have made their mistake? The key here is the transition from step 1 to step 2. What if we had subtracted 68 instead of adding it?
There it is. The friend's problem was that they had accidentally <em>subtracted</em> 68 to both sides when they should have <em>added</em> it.