Wow! Sounds like you've got a job on your hands Buddy
If the owner wants to keep the core of the pond the same, I'd definitely recommend installing an effective filtration system in it.
Now for the bad news. It's always better to start a pond off in the right direction, rather than having to retrofit it. Don't get me wrong. I'm not saying you can't install a very effective filtration system in a pond that was originally devoid of a filtration system. I'm just saying that this customer will more than likely have to live with certain compromises if they decide to have you install a filtration system in, or on it.
In order for me to be of much help, I'm going to need to know more about this pond. For starters, how deep is it, what are the dimensions, is it under a tree, are they keeping, or do they plan to keep fish in the pond? If you can answer these questions for me, I'll be able to give you better answers to some of your questions. Can you post a pic of this pond, so I can see what you have to work with?
Now on to the topic of algae. I hate to tell you this, but powerwashing is a bad idea. If this is a liner pond, you don't want to subject the liner to that kind of pressure. Bad things can happen if you do something like that. The algae is there because it has all of the conditions it loves. I take it that this pond is in a location that receives sun for a good part of the day? You've already stated that the pond doesn't have a filtration system. Since the pond doesn't have a filtration system in it to remove the nitrates, nitrites and ammonia that are in there (I'm sure she at least has some goldfish in there) the algae has an abundant supply of nutrients.
Let's talk about how to kill the algae now. You asked if power washing would be an effective means of getting rid of the algae. The answer is a resounding NO. If you want to get rid of the string algae, you're going to have to kill pretty much every last cell of that stuff. String algae is so persistant and hard to erradicate from a pond without chemical help, it's almost a waste of your time to try. You could spend hours upon hours cleaning the string algae from your pond, but if you left just one strand of the stuff, a carpet of it would be back in a week, or so.
To rid the pond of string algae (also called blanket algae), I'd recommend that you go out and buy a bottle of "Algae Fix." The instructions will tell you to dose your pond every 3 days untill the string algae is controlled. If my memory serves me correctly, the instructions will tell you to dose the pond once weekly after the algae is brought under control (I'd just use barley to keep it from coming back). It's a common saying in pond keeping. "It's not a matter of if, but when you have to deal with string algae." The "Algae Fix" will also kill the suspended algae in your water (the type of algae that can give a pond that pea soup look).
Just because you kill the algae with the "Agae Fix," doesn't mean your problems are over. To keep the algae at bay, I'd recommend using barley to prevent it's return. It will take approximately a month for the barley to become effective, but once it starts working, you shouldn't have any problems. The "Algae Fix" kills the algae and the barley controls the algae by introducing hydrogen peroxide into your water. The "Algae Fix" does it rapidly, in a stronger dose to kill the algae and the barley does it slowly as it sits in the water.
As far as a filtration system goes. There are bottom drains that you can install on top of the pond floor. They simply sit on top of your liner. They connect to a pipe that runs across the top of the liner and on up the wet side of the pond wall. This obviously won't look as good as an underliner drain system, but this would at least give them the benefits of having a bottom drain. If they have rocks all over the bottom of their pond, you can pretty much throw the bottom drain idea out the window.
Not only are there drains that don't require cutting the liner, but there are also surface skimmers that are the same way. Choose wisely when it comes time to choos a filter. There are some really nice ones out there, but there are also some worthless ones too. Be sure to get a pump that will flow the amount of water through the filter that the filter is designed to handle. Remember that too much flow through a filter can be just as bad as too little.
I hope this novel has helped you. Get back with me with the answers to the questions I asked earlier in this response and we'll take it from there Buddy.