so he boosted the car and said the battery was bad maybe?im gonna replace the battery and go from there.
So he boosted it and it started?
If No, go to
one of the GTA BMW specialists
If Yes, you need a new battery and you need to stop going to that..."mechanic" would be the wrong word... individual.
A huge number of ills are caused by a bad battery and ... mechanics
... who can't run a voltmeter will run around in circles changing ancillary stuff like voltage regulators and alternators and starters which appear sick when the battery is terminal. (Harh!)
Would still be smart to check the grounds. As Aviator said, when grounds go bad all sorts of bad, mysterious and intermittent things happen. Your car is a bit too new for bad grounds but it's certainly worth checking. You've already done the positive connections, right?
Please let us know how it works out.