Re: Tomato Sauce Whoops
I believe your friend’s advice is correct. The danger in not adding lemon juice comes from an environment that is not sufficiently acidic to prevent botulism spores from growing. In this environment, if there are spores present, they will grow and produce botulism toxin which can cause paralysis and death. Fortunately for you, while the spores are not killed at home canning temperatures, the toxin does break down completely.
So, if your canned goods have been growing botulism spores (and therefore toxin), reprocessing the tomato sauce will destroy any toxin that has been produced and adding the lemon juice will prevent the spores from growing again. If you are still nervous about the tomato sauce, just be sure to boil it for 10 min before serving.
As an added comfort, most tomatoes are acidic enough to prevent botulism spores from growing anyway; it is only some of the newer low-acid cultivars that are not. Here is what the CDC says about preventing botulism