Any folk on here qualified to tell me the odds of joining the RM Commandos after getting a hip replacement? Not for me but my mate who is in his 20s and still runs/cycles/hikes like crazy. Had it done 2 years ago.
What Soleil said, plus here's a link to Ninja_Stokers (An AFCO recruiter in the real world.) guide to medical standards.... http://www.navy-net.co.uk/Forums/viewtopic/t=11861.html
About half way in the first section are Musculoskeletal issues, which your oppo would be checked on.
Although it doesn't mention joint replacement there, it will obviously be a concern if there is a predispodition to problems in that area.
The odds are nil I'm afraid. Joint replacement in major load-bearing joint like the hip - not a chance of entry to any Service, not just RM. The risk of complications (eg fracture near the replacement) is high, plus as he is young he will likely need a re-replacement done at some stage in the next 15-20 years.