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It's hard to picture without an example of before and after, but here's my opinion:
If she just added a column and didn't alter the column(s) you used for joins, that's no problem - incremental should be fine (as long as you don't care about picking up the new column). If she adjusted the logic in both columns you use for the join, but they still match each other you will now have new data in the columns, and an incremental extract will only pick up the new rows based on your key field configured during the incremental refresh step. However, if you aren't joining on a date field, that means that old records have the old values, so if they should be joined with the new records, that's not likely to happen.
In my opinion, your best bet is to re-run a full extract to be safe. Tableau, like most analytics tools, is not very flexible when you change the underlying data structure beyond the contents of the columns themselves. If they no longer join in the same fashion, names change, or if datatypes change you're likely to get some weird behavior. If you have Tableau Server, you can push this extract to that machine so that it doesn't tie up your own computer.