First of all, to the extent that it's even possible, may this as non-personal (not about the you-and-her dynamic). That will be difficult, but keep it in mind as a goal.
What you need to do is make a list of the specific failures that threaten the project. Not the ones that "bug you", because they frankly don't count. You're not in a love relationship here; you have a job to do.
So make that list, and then prioritize it. Which of her failures is most critical TO HER JOB PERFORMANCE and TO THE PROJECT. Unless you're planning to fire her, then focus on no more than three of the top issues that you have with her. Explain, using specific examples and details that you've witnessed, what the problems are, and ask her to come up with ways to resolve those specific problems in measurable ways. (If you can't measure it, then it doesn't count.)
Regarding things like your foreign travel, give her specific tasks to accomplish: pick tonight's restaurant, for example, or set up our travel arrangements for your approval, or other defined and specified tasks. Make it known to her that SHE is responsible, and if she doesn't pick the restaurant, you're both going hungry, for example. If she doesn't make good travel arrangements, then you're both going to have a miserable trip. You can't afford to give her work ... and then re-do it yourself. Any rational person can see that. (And if she needs information from you regarding cost constraints, time required at various locations, etc., then don't withhold that from her to watch her fail.)
Again, you have to make this impersonal (no "it pissed me off when ..." or "I get upset because ...") and totally job-related. No emotion; just the facts, ma'am.
To sweeten it a bit, you can also compliment her on the things that she does especially well, but be sincere and honest about that, and again, praise job-related activities. Not something like "you dress nice, and people seem to like you, but ... you can't do the job". If you can't think of valuable and honest things to praise her for, then you may need to re-evaluate her job performance outside of your friendship: it may be time to let her go if she really can't do the job.
- Response by regnadkcin
, A Father Figure, Male, 56-65, Boston, Artist / Musician / Writer