Get a second opinion from an independent shop.

As long as it was not caused from overheating which can warp the cylinder head. There are plenty of fluid additives on the market to seal smaller leaks over time. Some are added to the coolant, some are added to the oil. Pending which way the contamination is crossing; oil in the coolant or coolant in the oil, or just a plain oil seapage down the side of the engine... will determine which one to get. Stick with popular brand names of these and read the labels carefully. Coolant in the oil is far worse than oil in the coolant.

And as long as you did not let it go for very long or have not been driving it many miles with contaminated oil to possibly cause bearing and journal damage, you may be able to save it. Change the oil first before adding these products to engine, they can save you from having to replace the entire engine or labor-expensive repair to replace the gasket. If afterwards, the problem appears to be solved, check for lowered oil pressure that could indicate possible bearing and journal damage may have occured from the bad oil.