Hello @Ed and welcome to our community.
A Duldung is issued temporarily (it could be valid for only a few days or up to a maximum of round about six months) and has to be prolonged every time it expires.
If circumstances change (e.g. the reasons for why he was granted a Duldung don’t exist any longer) your friend’s Duldung can either be revoked or Ausländerbehörde waits for it to expire. If your friend doesn’t hold a valid Duldung anymore he can get deported anytime. Since 2016 authorities don't have to anounce it in advance any longer.
He has a right to appeal against the decision of BAMF. Along with every letter he has received a so-called “Rechtsmittelbelehrung” (instruction on the right to appeal). It states at which court and until when he can appeal against the decision. Depending on the outcome of his court case his legal status might be changed (for example into an Aufenthaltsgestattung).
I hope this answer was helpful to you.