If tiles are still attached rake out loose mortar and repoint as per I did a few years ago https://www.diynot.com/diy/threads/does-this-look-like-a-poor-chimney-repair-job-to-you.655892/#post-6223596
Here is a bit of before and after pointing I did on my roof/chimney during covid lock down cos i was bored and had nothing to do ( 68 year old diyer at the time) I would expect pros to do just as good if not better
But of course that area of roofing was supported by 3 rafters and in essence it is now supported by 2 rafters albeit with lesser weight, hence why usual default method is to double up rafters each side of opening to give 4 rafters of support