I've done the Master Cleanse several times, the longest was 14 days. That's great that your friend is fasting with you, I know from experience that it's sooooo much easier to do with a buddy.
One thing I discovered was that if I kept the lemon juice to water ratio strictly at 2Tbsp lemon juice to 8oz of water, I felt the best. Not all lemons yield the same amount of juice, so measuring worked best. More or less juice in the lemonade seemed to create an excessive amount of bathroom trips or else made me feel hungry and weak. I don't know why that is, but the person I fasted with and I both found it to be the case. The maple syrup, I could adjust to taste without any problems.
Other than that, the thing I did on my successful fasts (but not on the ones that I broke ahead of schedule) was mix up all of my juice for the day at once. If I didn't, I'd get sick of squeezing lemons and mixing juice every two hours and lose my enthusiasm... and end up not drinking enough of the juice. That probably says more about me than the fast itself, but anyway, that's a strategy I found most helpful. I also learned (the hard way, stuck in a traffic jam and thirsty) to carry the juice around with me at all times in a cooler.
Congrats! You've already made it through one of the hardest days!
You've also inspired me to think about doing another fast soon. I'm overdue, I'm sure.