Generally speaking, health insurers will negotiate lower rates because they will have numbers on their side so can get a "bulk discount" so to speak.
That said, Euromedic in Dundrum, Charlemont and Northwood currently advertise an MRI price of €250 ([broken link removed]), so maybe Charter Medical have reduced their rates since. There are many private providers of MRI services around Dublin so it would definitely pay to look around first.
If your mate is a PAYE worker, he can also get tax relief on the cost, as well as GP, consultant and prescription costs.
Presumably, he's asked how long it would take to get an MRI on the public system. Just mentioning it in case he thinks he has to go privately.