I have a friend with a 28 Regal cruiser. The boat has some type plastic fuel tank. Don't know the material but its not metal. During hot weather, he smelled fumes, found the sending unit leaking and the tank had swollen. He replaced the sending unit but still gets some fumes. The vent hose is not blocked or kinked. My question is this. The tank wouldn't swell unless there was pressure in it and it can't build pressure if the vent is working, so whats going on? Any ideas?

I can't speak to the swelling issue without actually seeing the tank , but it's common for the plastic tanks to emit the odour of fuel. From what I understand it's quite normal and not dangerous.
A leak now ...... that's another kettle of fish altogether.

The smell might be coming from the vent. Depending on where it is located. On my boat the vent is located on the outside on the side of the boat. Only about 16" from the corner of the back seat. Under the right conditions fumes can be smelled, but never in the bilge.