1994 Buick Century

According to the garage, they say my son might have added oil to the brake fluid reservoir. The gasket is all swollen and won't fit on properly. The brakes are not working.
Does the brake fluid normally get black overtime or is it possible he did add oil? If so what does this do to the brake lines and what can be done to remedy the problem. The repair shops are reluctant to look at this. Would having a brake fluid flush remedy the problem or would other parts need to be replaced as well?