I'm sorry it turned out that your foal was lethal white. Such a shame--for the foal, for you, and for the mare.
I'm afraid that I would have to call--or write--and chew out the person who chose to do that breeding, but I don't expect it would have any effect on them. The person was obviously aware of what could happen--since you were warned about the possibility of the foal being lethal white--and quite obviously that person didn't care, since he/she chose to do that breeding anyway.
I know there have been some that do that cross, LWO+ to LWO+, and they claim they only get a lethal foal 10% of the time....like that justifies the practice! To me even 10% loss is unacceptable, and the fact is, each foal from that cross has a 25% chance of being lethal white, even if it's generally "only" 10% that actually are homozygous for LWO. I've never understood why these people think it's necessary to do that cross. I know one person on here on time said that they do it that way because if you breed a LWO+ to LWO- there is a 25% chance the foal will be a solid. I guess those people figure that 25% chance of dead is better than 25% chance of solid colored???