Is it a requirement of your probation to have a job, or do you just have to show interest in finding one?
Did you have a private attorney when you were in court for your case? If you did, then I would call this person and ask him basically what you have asked us. S/he's probably used to these types of situations.
After this, I would (politely) explain this to your PO. Your PO wants you to have a job first so that you pay your probation fees and second so that you can work toward a better life. S/he's probably not going to do anything to jeopardize his/her chances of getting your payments. Sometimes POs will make physical visits to your place of work, sort of life secret shoppers, just to see that you are actually there. I would tell him/her exactly how this all progressed and plead with him/her not to call your boss.
Or you could do what I did. Tell your boss that you have collections agencies after you and that they might be calling to try and get information on you. Tell your boss that you want to keep this as private as you can, and for him/her to only really confirm that you work there, and as full/part-time. Here, you are hoping that the PO does not reveal him/herself, but if the call is going to happen, it's worth a shot before you get fired. That is, if you're sure that you would be fired if the truth came out.
It's really unfortunate when POs want people to have jobs, and then go and do things that could cause people to lose those jobs.
