Sorry, but she can be held for a total of 24 hours actually. She can initially be held for 8 hours, then if she hasn't been charged she can be held for a further 8 hours with a senior officer's permission (inspector or above) say, if further investigations are necessary. Another 8 hours detention can be granted again up to a total of 24 hours.
Then, if necessary she can be held for a further 12 hours on the authority of an officer of the rank of superintendent or above.
Also, police stations are not obliged to say who or who isn't being held there.
Your friend is entitled to let someone know where she is though. The custody sergeant will make a phone call on her behalf to whomever she wants to know.
Shoplifting (which is really just theft) is generally dealt with fairly quickly because it�s usually an open and shut case (due to CCTV, security guards, etc) but I wouldn�t expect her to be �processed� faster than about 4-5 hours. She�ll probably be out by midnight or soon after unless there�s a complication.