score:2
If she is:
she can get a visitor in transit visa. She can also get this visa if she is traveling to/from or has a visa/permanent residence of USA, Canada & a couple of other countries.
Technically you can get this visa on-arrival at the airport as long as the immigration officer is satisfied with the mentioned conditions.
However, if you are not sure about it you can apply for it beforehand. The fee is 62 pounds.
PS: As far as I know the 24-hour direct airside transit visa (DATV) is if you won't leave the airport.
Upvote:2
Check this pages:
Visa to pass through the UK in transit
But, according to this site, your friend may need a Visitor in Transit visa instead.