Cat delete on ML350 blueTech - 2011

I hope I am asking in the right place... anyways, I have a buddy with a 2011 ml350 with the cats now hollowed out. He bought it like that but ran into issues.

I was going to buy it for my farm as it would be fine to bomb around the 200 acres and he will give it to me dirt cheap.

However, somebody did program or reprogram this thing. I had it for a test drive, but after a few starts... maybe 6? it won't run anymore and needs to be reprogrammed I'm told.

Does anyone know what can be done, if this can be done correctly, or is this just going to be a huge nightmare. I'm not sure if used cats could fix the program, my guess they are a fortune if you can find them and I'm in Ontario Canada.

Any advice or help etc would be appreciated.