It's very hard to say what has happened here.
I wold be curious as to the qualifications of the shop you took it to. Hopefully it was a BMW independent that knows BMW's inside and out.
BMW's are money pits...I know. I have one and have owned two others. It's quite possible your oil leak has noting to do with the work they did. It's easy to spend 2500.00 only to have another big repair come up the next month.
I'm not sure if they have to pull the water pump in order to pull the head but someone else here might know.
If this is the case, they should have looked the old water pump over carefully to check for impending failure although sometimes it's impossible to tell.
You might want to seek out a second opinion from a shop who specializes in BMW's!