You can send the letters to the PO but I strongly suggest you talk to the Defendant's lawyer first. You cold possibly say something in the letter that could inadvertently hurt the defendant rather than help him. Most lawyers are happy to have information that is beneficial to their client. The lawyer could review the information prior to it being given to the Judge. Whereas if you send it directly to the probation officer, it's possible it would end up in the report before the defendant or his lawyer saw it.
Nothing in this communication should be construed as creating an attorney-client relationship. I provide this service for educational purposes only. I will take no action on your behalf unless you have hired me and a written retainer agreement is signed. I am licensed only in Tennessee and I strongly suggest you consult with an attorney in your city and state as Statute of Limitation deadlines can limit your recovery.