wouldnt worry too much!
about 1 in 5, 5 year olds arnt dry at night (1 in 10 10 year olds as well!)
im sure she REALLY wants to be dry, but its not worth sleepless nights and sore skin over...
if she doeanst want to wear a pull up you could try using bed mats so she doesnt feel so self concouse...
to save time at night why not do:
waterproof sheet
cotton sheet
bed mat
cotton sheet
(i find bed mats wriggle out of place so you change the under sheet anyway... buy putting on top as well you at least have a dry sheet underneath as well when other is whipped off!
If she is having 2 + accidents a night add another mat / sheet layer...
or else in est in a few of those sheets that are water proof with a terry backing, and just put a few on the matteress, so you just take the wet one off!
you are right.... it may be disheartening if she gets no stickers...
Maybe a freind for tea is not the right reward?...
Generally rewards are effective if they ONLY get them for doing what it is to get a sticker....
therefor if freinds come for tea any way (as they will surely?, you wont go for weeks with no friends will you) then it makes a mockery of your sticker chart.
It might also be seen as a 'punishment' if no friends are coming, and what you really want is a reward!
perhaps you could have 1 friend a week and an 'extra' friend for a drynight? or choose a different prize
(you could by a lego / polly pocket / tea set or what ever toy, and get one element of it for each dry night, so after several dry nights you have a new toy)
She sounds like a bright child... if she is physically capable im sure she will be dry soon...
but you may have to accept that she is one of the (VERY) many children who develope this skill later, and either accept 'nappies' or find an easy way to cope with accidents for all of your sakes!
Good luck!