As sad as it is this is just nature taking it's course. It is very sad and nature can be cruel. Although the Jays seem like evil birds ransacking the blackbirds nest they are just trying to survive too, they probably have their own babies to feed.

However she could try putting some other food out that the jays would eat instead. All animals will go for the easiest option when it comes to finding a meal. If there is food lying on the ground they will go for that rather than endure the difficulty of raiding a nest, using valuable energy fighting off the blackbirds or waiting on the blackbirds to leave so they can steal the eggs/babies. If the jays have another food source then maybe they will leave the blackbirds alone. I don't have Jays that come to my garden but I have other types of crow and magpies too and they seem to eat just about anything, we always used to put our leftovers out for the crows, they love bacon but you have to cut it up small.

My Mum loves the Blackbirds that nest in an ivy-covered tree stump in her garden. She feeds them and watches them prepare for their family, then is heartbroken every year when the Jays arrive and kill their family. She's put fencing wire around the foliage to no avail.