Frist 13 koi in 90 gallons? You should tell your mothers friend that is 13 too many koi in there. I have 4,000 gallons with 17 koi and I am overstocked by 1 fish. Please remember, koi are dirty more so than goldfish. You should have a min. of 250 gallons for each koi. Also a koi can easily grow to 24 inches and some will grow to 36 inches.
I am sure that your moms friend has a beautiful pond, but koi aren't for a small area.
You can make your own filter, but will have to buy the pump. The best filters are the home made ones anyhow. They generally provide better biological filtration and IMHO much easier to maintain. Since you are looking at doing a small pond what you can do is get a submersable pump to put in the bottom of your pond and run a hose to a rubber made container, have the water come up from the bottom so it is forced through the filter media you choose. I would get some filter matts or something like it plus you can get some filter media fairly cheap. You'll want to fix the rubber made container so that it will spill over into your pond, this will help oxygenate your water while giving you a nice water fall. Make sure that you at least cycle your water once per hour.
Here is a link on how to make a sponge filter
http://koivet.com/html/articles/articles_d...name=Filtrationhere is a link on how to make a barrel filter, remember that this article was writen with a large pond in mind. You certainly can make a filter like this for your small pond on a smaller scale.
http://koivet.com/html/articles/articles_d...name=FiltrationThey sale different types interesting homemade filtration media on ebay. One that I thought would work great is pvc pipe shavings. This is something someone could make but I'd say it'd be time consuming. For the size of filter you need it probably wouldn't take but a couple of hours? Would provide a lot of area for your good bacteria to grow!
Another great resource for the DYI ponder is Roark's Experimental Puddle. They have lots of stuff that could help you on your way.
http://www.click2roark.com/cgi-win/weborde...document=index2Please keep us posted on your progress. Oh, and remember, pictures get a GREAT response, we LOVE them.
Scott