Dude,

If they dented your frame then you should talk to them about it. Explain to them what you know of what happened and what you would like them to do for you (this is important). Don't speculate. Just give them the facts like, "the bike was fine when I gave it to a woman at the ABC Airline check in. When I got to my destination and unpacked it, I immediately saw the dent." Don't put any assumtions in there because they may call you out on it.

They have insurance to cover events like this. Just write a nice, well thought out, pleasant letter with as many details as possible (flight number, dates, when you saw the bike before and after the dent, etc...). Include detailed photos and copies of your ticket. The sooner the better.

Explain to them that it's more than just a blemish, it is part of the structural integrity of the bicycle. Especially since it is aluminum. Also mention that it cannot be repaired. Maybe say that you would like either another identical frame or cash for you to buy one yourself. Let them know from where to get a new frame for you or how much cash it would take them giving you to replace it.

Remember, they handle stuff like this all day, every day. Just be nice and level headed about it and things should work out.

Good luck.