This turned out to be a long question, and several considerations needs to be taken into account in order to give a proper answer, but I think the essence of the question is good and I believe that all of us have faced situations like this.
The problem here is two-folded. First of all that the project started out on the wrong foot - with no UX plan. How is it possible to get the project back on the right track? The second problem is about getting access to real users. What can be done without real users on hand?
The clients attitude
The first step is to bring the right point of view to your friend. To succeed, it is quite essential that the right person has the right attitude with respect to UX. So this is one of the first considerations. Is your friend just small talking, as in "Oh, you're an UX expert, hu? What you think of this spare time blog of mine?". Or is your friend really serious about the effort that should be put into this, as in "I really should have focused on this before, but one thing led to another and here I am - and I really want you to help me to deliver a killer solutions!".
To succeed, it is essential that client/contractor is in your boat.
First aid - DIY
Then, as a first aid "do it yourself" solution for the client, you should ask some judicious questions. By asking the right questions to your client, he is forced to make some considerations and thus be pushed towards an UX centered thinking. Having pointed his mindset in the right direction can be critical when design decisions are made.
1) Who is the main user group?
2) What is the main purpose of this site/app?
3) What is the main task for the average visitor?
4) Can you describe the purpose of the system in one sentence?
Give the right person the right mindset.
But what can I do
Well. There are tons of things that can be done - even without any users. You have a bunch of methods that are called "usability inspection methods". They are carried out by an expert (or a team) and they don't require any users - just a product (fully functional product, competing product, deprecated product, prototype, sketches, mockups, wireframes, etc).
Take a closer look at:
To conduct the first two, it is very handy to have some guidelines to work with. The GOMS-analysis is very interesting, but requires some training and experience.
Perform inspection methods to avoid the most common pitfalls.
Some things should be done anyway
It is quite common to perform several UX-related activities after the product is released. And there is no reason to believe that these activities become any harder to conduct. You just need to pick out the appropriate ones, adjust them and adapt them to the situation you're in.
- Follow-up interviews
- Logging of user interactions
- Documentation of support and userfeedback
- User testing
Do whatever you would have done with any other released product.
OK, let's take one step back as well
The obvious problem in these situations is that there is no UX plan. The customer believes that UX is a wrapping that can be applied to the solution right before it is released. We know that this isn't true, but we also know that it is never to late to put UX on the agenda. Therefor, you still need to establish a plan. Conduct a quick-and-dirty analysis phase where you gather some basic information and establish some essential goals. In the eyes of the owner, the product might seem completed. But as an UX expert you need to do a proper job and enter the project with blank sheets and a very thoroughly prepared prototype :-)
Consider the first, released version to be the first iteration of the design phase and carry on towards the second version as you normally would during a second iteration.