This is not really a pro bono case -- either your fiancé cannot afford a lawyer and a public defender will be appointed, or he needs to hire a private lawyer in Colorado.
Why not the Public Defender?
This is not intended to be legal advise or as legal representation. I am a California personal injury attorney . Be aware that every state has its own statute of limitations; and statutes & case laws that govern the handling of these matters.
Since the public defender option is available, most pro bono organizations do not offer criminal help. I'm sure the public defender is appointed. If you aren't happy with the PD, hire private counsel for a fee. Those are your options.