Is that really okay? I mean what the chemicals?
In the Yaoi world of fantasy and fiction anything can be lube [and if it can be used, it most probably was used] - but in the real world specially designed silicone-based or water-based anal lube is the only healthy way to go. Even oil [or oil-based lube] is bad since it forms a layer on the skin and can cause bacterial infection. Silicone-based lasts longer & isn't absorbed through the skin, but many people are allergic to it. But yaoi is a special fictional world where these things don't matter ^^.
Shampoo does that (burning, etc.). Conditioner does not. (I have experience here. I've used it as lube for anal.)
Conditioner has chemicals, sure... but so does commercial lube (it is, in fact, nothing BUT chemicals). Mind you, in the bath/shower, silicone lube IS the best. However, I (and many of the gay men I know) have used conditioner when lacking anything else.
Either way, I don't think the shampoo was used as the lube.
(Honestly, a lot of yaoi doesn't use ANY lube. *cringe*)
It wasn't shown, but did they use the shampoo as lube?! It keeps showing pictures of it while and after the scene