Legal Question in Business Law in California

Since you're given your friend a reasonable time to pay and he hasn't done so, your only recourse is to take him to court. Hopefully, the repayment became due within the past two years which is the statute of limitation for oral agreements. You might make one last attempt at, perhaps, suggesting a repayment plan which would be more palatable than paying the full amount all at once. The e-mails would be evidence of the fact it was a loan and not a gift and that he acknowledged that the amount was due at that time. Good luck.