Let see-
1. Make sure she is wearing the most comfortable footwear as she will be walking ALOT!
2. Tell her not to impluse buy. If she see something she liked, look around at the other stand first, chances are one of them will have the same item the first stand had but it is cheaper. E.g. I bought my Miku Hatsune Nendorid figure for £20 at the Forbidden Planet stand where Tokyotoy sold her for £39!!!
3. Granted with 2. some items will be limited any may not have the stock to replace it (usually the handcrafted plush) so it;s really your own judgement if she want to buy it asap incase it sold out or look around to find a better valve.
4. Speaking of TokyoToy, don't buy anything from them, they are usually expensive as hell and I read that some of the plushs they're selling are knock off.
5. Bring packed meals since the food served/ sold in the con are expensive.
6. Have plenty of water/ drink since it will get hot in there!
7. Depending if you had ordered the ticket online or not, chances are she will be in the queue for a while. She should have something to pass the time like a Nintendo 3DS, book orthe phone got access to their wifi.
8. Speaking of the queue, you may either want to get there early to avoid being at the queue during openning time (you're in there faster if you were there early) or get in the afternoon when the queue is shorter. Also you could go there on Sunday to get a shorter queue but at the expense of less stocks on sale on a Sunday.
9. Bring ALOT of cash with you (like £100)! Yes they do got ATM inside (and a few stands may take online tranaction) but chances are it's the paid atm (pay to used it) and there will be a queue for the ATM aswell. Last thing you want is you're short on cash and then you have to look and wait to use it like I had to enure a couple of years ago with a mate...
10. Make sure she had read the cosplay rules on their website.
11. Check the forecast on that day. Sure you should be ok since last time I been there years ago the queue was indoor unlike the Mancehster MCM but it's best to be prepared (I had to queue outside in the rain for 2.5 hours but they were selling desposable poncho at the queue).
12. For effort points, she should know the character she's cosplaying like knowing the character line or two and pose like the character too for the photo.