Air is not a good idea and compressed air is downright dangerous, compressed air from most sources will contain suspended oil. Air blown in by mouth will have bacteria. When the condom or balloon breaks or leaks, and it will, you will undoubtedly cause your friend some sort of harm.

Pressurized water is inexorable and you will likely cause a rupture or other injury.

Now I like tight, but if you must, I suggest a speculum. Made specifically for the job of enlarging orifices for inspection, there are models that can be held open mechanically...