I think this is the key. If the plane’s going anyway, the additional environmental impact of you being on it or not is essentially moot.
As for whether the plane should be flying in the first place, well obviously not.
If it helps you rationalise it at all, if anything you’re making the plane more efficient by essentially halving your friend’s carbon footprint.
Alchemy! Now this is the out-of-the-box thinking that I like!
In all seriousness, lead is lead because it’s made of lead atoms. It can’t not be lead. (The reference to alchemy was because before we knew about atoms, many alchemists tried their hand at turning low-value metals like lead into high-value metals like gold).
To answer your question in a silly but scientifically accurate way, there is a temperature to which lead can be heated to become something else, but these are nuclear fusion temperatures, like you get in the Sun.