DTDM

Hi,
If he is your good friend then he will understand well what professionalism means. If it is hampering your work then you can hire a different content developer who can give you the content.

If you keep messaging them all you will do is make things hard for yourself with most clients. Remember the world today is connected and online, you apply to much pressure and they will tell the world online without being truthful! One bad comment online will cost you 100 customers! 1 good comment online can help persuade 1 customer. The odds are not stacked in your favour so giving 'reason' by applying pressure is never a good idea unless you have the ability to back it up in court. No contract - no court!
 
You described them as a friend, then I'd suggest going to see them. If they thought they was only bouncing ideas of you and have signed no contract you are already out of pocket but can walk away lesson learned. Or they might have a genuine reason, maybe their business idea is not viable and they don't wont to sound stupid. There are many 'legitimate' reasons in a clients eye's that we the 'servant' just can't see. (Or at least that's the way they will sometimes think). Talking face to face or even over the phone will often clear things up and get money moving.
 
I noticed you took a deposit but said it 'was not paid in full' - Alarm bells would be ringing in my ears if it was my own brother let alone a mere friend!
 
Business is business, you are not a person that has worked for your pay day from an employer. You are a business, they are not people out there, they are clients with fat check books. When you speak with your client be a business not a person, understand their difficulties and misgivings but business is business and you need to keep a roof over your head!
 
One bonus is that the design and build is already done, next customer that wants something like this project will take very little work, time, and effort to please them with a nicely polished and finished product. They will probably even go and tell somebody how quick and efficient you was. So not all doom and gloom.