A friend put a 4" sani-tee in backwards. A 2" drain from the tub and sink comes into it. It's going to be hard to get to to turn around. Could I leave it the way it is? The 2" line drops in from the top with a 90 elbow.

Correct position of San Tee will assure proper flow. Yes, it will work, at least for time being, but eventually it will inhibit flow and you will have to deal with it. Also, if you insert snake into to pipe to clean it, it will be directed upward, instead of down the pipe, accomplishing nothing.

If it was my house, I would fix it. If you have limited space to work in, you can always use shielded couplings, or Flex-Seal couplings to make all connections. It is a 15 minute job.

Hope that helps

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Milo