I use my ER6's only on airplanes (and a few times at home or in a motel). I don't even use them in the airport terminal -- they just seem that vulnerable to physical damage. Having said that, and although I find them almost too bright-sounding, when a good seal is achieved, they really do isolate and block out the ambient noise in an airplane cabin very, very well. And it is fun to hear things in your favorite music that you hadn't heard before. Just not for fans of fat, funky bass. I'm not sure I'd use them on a subway or bus, though, and I can't imagine having them as my only, day-to-day set ... but maybe that's just me. Just be sure your songs are at a sufficiently high bit rate, because the Etys are certainly revealing.
I also have the Shure E3C's, and they are so much sturdier than the Etymotics. With the foam ear pieces, they're comfortable and they isolate on airplanes and in airport terminals very well, if not as awesomely as the Ety's do with the two-flange ear pieces. I just think they're a more natural, comfortable, and all-around better experience than the Etymotics ... these could be excellent day-to-day 'phones ... but you need to hunt for a good price to come even close to your price range.
The Westone UM1's sound intriguing, but I'll have to live vicariously through others' experience with them -- at least for a while! I hope to read more about them.
I hope your dad is happy with whatever he gets.