Your friend seems to have developed a complication called Myositis ossificans. Reduction of range of motion is one of the early signs of this complication.
Vigorous passive movements, passive stretching and massage are contraindicated in such cases.
Basically, he should rest the elbow for 2-3 weeks. After that, active exercises with roller skates should help in improving the range of motion of the elbow joint. The regime of exercises is progressed towards achieving full range of active movement and functional use of whole upper extremity.
Surgery is rarely indicated in Myositis ossificans. Surgical excision of the myositis mass is undertaken only after the radiological evidence of its consolidation. It may take 6-9 months for the myositis mass to consolidate. Since there is a high incidence of recurrence, the role of surgery is controversial. It is usually undertaken if the elbow is ankylosed in a functionally unacceptable position.
So, I suggest, your friend should concentrate on the exercises keeping the above in mind and aim for the best functional range of motion he can achieve and not force the pace at the moment.