he's out of the country almost every other week and when she returns, she requires she goes celebrate her return. So, all of these add up and I find myself 'celebrating' her by paying the bills as it's always her day.
If she's travelling that much then she can afford to pay for herself.
Did you post about this recently - about being stiffed over for an expensive dinner?
If you want to keep the friendship, then you need to either have a conversation about it but in a calm and neutral way without accussing her of being a money-grabbing manipulator. Or grasp the nettle when the bill comes and start splitting it whatever the culture is. If she is difficult about splitting it, then you need to pre-plan what you are going to say and say it and stick to it - and say something like
"I'm happy to split it but if you are insisting not, then I believe its your turn because the last (fill in her how many are true - two or three times) I paid when (identify where you went and when like in December when we went to the Ritz and in October when we went to the Alchemist or wherever)."
and just keep repeating it. You will need to go forarmed with toughness and don't buckle! Either the bill is split or she pays it all these are the only two options.
Again if you think the friendship is worth keeping, when she pays for theatre tickets and says dinners on you but books somewhere expensive - either say to her I'd prefer to pay you for the ticket and split dinner because it's fairer OR just cancel the expensive place, rebook somewhere you want to go and then tell her. If you are feeling evil, do it at the theatre after the performance ' oh Iforget to mention - I thought it would be better to go here as its closer and cheaper".
Personally all sounds like a right manipulative drag and I'd find other friends but I know sometimes even crap like this can be worth putting up with if a friend is convenient or you are otherwise lonely or dont have any one else to go out with in a particular location.