I'm going to demand my friend pay me back the money he owes me by a specific date.
Why would you demand a payment from a friend?
Because he owes me a lot of money, and I want it back.
I understand that it's your money, but if your friend is in desperate need of it, you should give him some time. Explain to him how you're feeling. I'm sure the two of you can work something out.
It doesn't matter what he needs the money for. He owes me.
You're right that he does owe you money, but he's a friend. Friends should be willing to be patient and respectful to each other in difficult times. If you really need the money back, you could consider just asking him for small payments of it on a regular basis.