1 – A psychologist – The field of psychology is huge and the life coach would have to be quite careful that they recommend the right type of professional help in this area. If for example a client is exhibiting what might be considered abnormal behaviour (in some defined sense such as statistically or morally) then it could be the right thing to recommend seeing a clinical psychologist.
I think it is probably unwise to be specific about recommending any particular type of psychologist unless the life coach fully understands the difference between them and is in some way qualified to make a recommendation.
It might be appropriate for a life coach to suggest seeing a professional who specialises in positive psychology. This area is probably the most aligned with the principles and guidelines that sit behind life coaching.
2 – A GP – If the life coach believes that the client is suffering from some physical ailment
3 – A Social Worker – Again, the life coach needs to tread carefully before recommending this type of contact.
4 – A priest – It may be that the client is confused about / doubting their religious beliefs which may be preventing them from moving forward in life.