If she's (you've) paid for a seat, then she will be allowed the same luggage as any other passenger ...
New rules mean that carry-on baggage for all flights taking off from UK airports must have a maximum size of 56cm x 45cm x 25cm.
However, there is now no limit to the weight of bags that passengers can take with them on-board.
I think cases in the 'hold' however, vary between airline & destinations, so you'd be better off phoning the airport for exact amounts