It feels like such a considerate and sensible thing to do. But now she asks me to hold her hair while she's sucking his dick, wat do?
That is nice of you. Hopefully she repays the favor with your lover.
Holding her hair? Are you having a threesome? If not, just provide hair ties or clips or something. Holding someone's hair while they are giving oral to someone else runs the risk of pulling their hair.
It totally makes you a nice guy and it's a hot thing to do. The hair tie is a good idea but if they are being nice enough to let you be involved, you should gladly hold her hair back and even help her suck his cock by guiding her with her hair.
And I would bring cookies and condoms for my guy and his lover. See, I am nice too.