Hi MissC,
I think you're wise to discontinue the use of cod liver oil for now. It's obviously upsetting her tummy and she'll be miserable at having accidents in the house and feeling off colour. No fun for you either! ;)
Another concern is that CLO is safe to give to dogs but only in certain quantities. It is rich in Vitamin A and is found in most pet foods. If the combined amount is too high then it can be toxic for the liver.
A couple of alternatives: Firstly, your aunt could try giving her a small tin of pilchards or sardines once a week mixed in with her food. She'll still be getting the occasional fish oils which will help with her old joints and it'll make her coat glossy too. Secondly, glucosamine is now recommended for dogs to help ease arthritis and joint deterioration. I've heard of a number of dogs who had improved mobility a couple of months after they'd been given it. It seems a while to wait for the benefits to be felt but well worth persisting with.
I hope the poor old Lab settles in the next 24 hours. A small dose of Pepto Bismal (5mls for a Lab.. and yes, it is safe to give to dogs but NOT cats) should settle her tummy and stop the runs if it still persists in a day or two.