It all depends on what flavour of Automatic Speedmaster as there are several, The compound movement models like the reduced are a bit of a pig to get sorted as many independents won't touch them. It is the DD module which causes issues, Omega themselves simply exchange it out. If he has a 7750 derived auto then most independents would be fine with that so that is your best hope.
There are also models such as the broad arrow range using the 3313 co-axial movements which AFAIK can't be serviced by anyone other than Omega themselves, even STS won't do them. Same goes for the more recent 9300 models. The earlier pre-coax 3303 may be easier to get sorted. In short whatever he has, it may take a little work and effort to find someone other than Omega to do it and either way it probably won't be cheap. Omega charge a flat £520 to service a modern chronograph on bracelet.