These fish are too big and too agressive to be put together, even for a 180. The max size on PB tangs and
purple tangs are 8-12", and naso tangs are 12-16". Even if the PB and the purple tang do not kill each other because of territorial disputes, you're doing the fish a disservice by keeping them in an area so small. If your friend was setting up a 300g, you might be ok, but a 180g is simply too small. No amount of experience or water quality will make up for the fish's natural needs for space and territory.
We are concerned for the sake of the fish on this one. Please be mindful that these are living creatures, and treat them accordingly.