Perhaps. The rapid temperature change from the hot bathroom to let's say out side in the winter or a cold air conditioned room would concern me more. The potential for condensstion inside the phone would be present. So is he right, probably.
I usually have my phone on the sink so I can listen to music or Howard Stern while I shower. My moms boyfriend claims the steam from the shower can ruin my phone and/or my battery. Any truth to this?
This could happen. With the size of the water droplet in the air from the steam could very easily get into the small places on the phone. Just think of this; with the moisture that clings on your mirror and walls is very well happening to your phone. Not as much because it is just under where the steam hovers, but as it cools it will drop.
Now I don't think it is a very coming thing as I have never kept up to date with the issue but I can see it happening.
I think what is more likely to happen (not saying actual water damage is not going to occur, depending on your ventilation and humidity in the air) is that the water damage sticker inside the phone will display water damage from the humidity condensing on the device, thus your warranty is void for future claims.
This is when a zip-lock bag will save you. I have iHome mini rechargeable speakers that work great and the phone stays in the bag, safe and sound. One little wire comes out at the top of the bag and the rest is zipped tight.
Here is something to think about when in a humid to wet environments. Unless your phone is qualified to at least Mil Standard C, like some Nextels especially the i355, you can throw that phone at a wall and it will be fine, but the wall if sheet rock might have a impression on it. Should not be introduced in that environment meaning, keep it away from moisture.