How can I recover a reasonably well documented personal loan from someone out of state?

You can sue them in California and probably in Montana.  The question is whether they have sufficient contacts with Colorado so that you can sue them in Colorado, and I can't answer that from the facts you've set forth in your email.  Some factors relevant to this issue are whether they have any property in Colorado, whether they regularly visit or conduct business in Colorado, whether they have communicated with you about the loan in Colorado, etc. If I were you, I'd also be concerned about the statute of limitations.  As I understand it, you loaned your friend money which he/she agreed to repay when his/her personal injury claim was settled.  Since you know that the settlement amount is greater than the amount you are owed, something you couldn't know until a settlement had been finalized, the claim must have been settled, and the clock is ticking.  I do not know the Montana statute of limitations for breach of contract (I assume that Montana's law will apply to loans you made while you both resided there), but I am aware of some jurisdictions where it is as short as two years.  If you sue to recover too late, some or all of your claim may be barred by the statute of limitations.