Not holding charge

[cl55 amg]

After a bit of help for my friends Astra, The problems are as follows;

Car wouldn’t start called the AA out to it and they said my alternator had broke​

Had new 1 fitted and after about a week it failed to start again,called out the AA and they said it was a loose connection from the alternator to the battery,he fixed the problem.​

Approx 2 weeks later it failed to start again,called out the AA they said your starter motor had gone,so I had a new starter motor fitted ​

A week later would not start again,had the starter motor replaced ​

A few days after failed to start again and auto electrician looked at it,he took a relay out of the car saying it was draining the battery​

A few days later failed to start again,starter motor was changed for a vauxhall 1,​

A week later dials starter jumping up and down the abs light,the traction control light and the air bag light was coming on and off plus the power steering was failing(meaning the alternator is not charging I'm told) failed to start again...

I know its not a Merc, but hoping somebody might have some pointers for him to check.

Thanks Danny​