Ask him to put that in writing and have him show you where BMW advises against it.
A square set up with 275/50/19's will work just fine. The key is that the diameter of all the tires are the same. Your speedometer my be off slightly. Stock 19 inch wheel sizes with the sport package are 255/50/19 or 255/50/19 front and 285/45/19 rear.
When you buy your tires check on these main things:
- Load rating
- tire diameter (critical if going with a non standard size)
- width - you can go wider or narrower; but, there are limits based on rim width and clearance in the wheel well.
He just wants to seel you a $5500 winter tire package.