It can be difficult when dealing with an employee upon whom you depend because they are competent, but on the other hand are not shored up in the area of manners and the etiquette of your office environment. My approach would be two prong, first of all, I would be very vigilant about finding a possible replacement for this particular employee, in case you need to eventually make that unfortunate move, and at the same time, I would begin an Professional Improvement Plan with said employee to help make them keenly aware of my concerns and expectations as an employee of our company. Starting an improvement plan with the employee will express the seriousness of your concern while also giving the employee specific areas to work on and a way for you to observe and measure if there is indeed improvement.
You can download a Professional Improvement Plan online. Hope this helps....All the best.

Business relationships resemble love relationships in that trust is imperative inside that relationship. Ask yourself, is this a common pattern that you have always seen with this partner, which you have chosen to ignore? Is it a recent development which could be connected to some personal issues he/she is going through? Finally, is this a very rare hiccup that has occurred but you have always been able to count on this partner in the past. Analyzing these three perspectives could help you to decide how to proceed from here, but do also be aware, if something underlined issue is brewing between you and the partner or this partner and someone else, he/she could be using a passive-aggressive behavior designed to sabotage the project in order to humiliate someone in the company. Try to explore as many perspectives as you can...keep a journal of your thoughts and possible solutions. All the best