Extreme vanos failure can lead to metal parts being sent around the engine which is never going to be good and will most likely result in an engine tear down. Not saying this is whats happened but if an E60 M5 engine did need to be opened up it's going to rack up a nice bill in part and labour as there will be parts that would be foolish not to replace when engine is apart.

First things first is to get it diagnosed by a reliable and reputable source, if your cousin is Dublin based or near Dublin then get him to take the car to Motor Confidence.

Hope this helps,