Cats identify each other by smell, not by sight. If they don't smell like themselves, she's going to treat them like strangers and hiss.
I once had to wash one of our cats, because he got grease on himself. When he ran up to our grumpy cat, Tasha, to play...she whapped the heck out of him and gave him an earful of yowling, too. (Up to this point, the two of them had been fast friends and playmates). They finally got reacquainted, after we held him and got our smells all over him again.