Water pump hex bolts

I have a problem which I hope someone could assist me with. First, I bought my very first Mercedes (used) 3 weeks ago and I still think I got a good deal. my problem is that I knew that the water pump needed replacing. I looked at it and said I could dod this. What I wasn't told, found this out tonight, is that they tried to change the pump and rounded one of the hex head bolts out on the bottome of the pump.
How can I get it out? Couldn't afford the $360 plus tax for labor to replace the pump in the first place, not including the $282 plus tax they wanted for the pump.
There has got to be an easy, safe way to get that bolt out. Dreading the answer......