If you invoiced according to the contract and weren't paid and are not receiving replies to communications your friend is a deadbeat. I suppose you could lawyer up. That's a medium term strategy. The short term should be based on the assumption that you will never be paid. Some people are just assholes.

The way to reduce this sort of risk is screening clients and requiring retainers applicable against final invoice. And no favors for friends. I mean the reciprocal of you lowering your rate or being flexible on terms is your friend paying more than the standard rate under terms more favorable to you. Somehow that is not where people usually come from.

Don't be anyone's bank.

Good luck.