If your friend wishes to make a career as a yacht engineer, he needs to apply to the MCA for a Letter of Initial Assessment. This will tell him where he stands on exemptions he may have due to his mechanic qualifications and experience to the commercial yacht equivalents.
If he takes a job as a deckhand, any seatime he accrues will not count towards seatime needed for onward tickets unless he obtains a job as a Deckhand/engineer when he will get half.
Worst case he will have to start at the bottom of the ladder and do an AEC course. He won't gain any employment on a commercial vessel as an engineer unless he has at least this.
If he weights his CV towards his mechanic experience he may be able to get a job on a privately run yacht, but there is a lot more to being a yacht engineer than just repairing equipment within his comfort zone, eg. fresh water managament, sewage treatment, refrigeration, ballasting.
Gaining some daywork employment may help him to get some experience in the layout of engine rooms, meet some personalities and generally network.
I Hope that this helps.