Very strange WiFi problem, reward for everyone who helps me!*

Hello community, I have very strange problems with my friends Wireless (Yes, i am only "hacker" around here, so I have to fix everything that works on electricity, including washing machines) anyways, He has internet at home (ZyXel modem/router newer version, dont remmember exactly which model) and WiFi, I can connect everything up there with no problems, but he wants internet in his office which is about 70-80 meters over the parking lot. First thing i installed was TP-link wa-5210g, I used it many times before and I am really satisfied with it. I put it into his office in hope to connect to his router at home, couldnt find it. Okay, not a big deal, router has weak WiFi antenna anyways, so I moved TP-link to his home, put it onto balcony fence and pointed it directly to his office. (there is slight angle from where you can see the office door) went out with my laptop and tested it on parking lot (number 4 in picture) 5/5 signal, everything as expected. next up stood in front of office (nr. 2) 4/5 signal, still no problems. At last I got into the office and I was so happy to see 3/5 on my laptop, but not for long. Connection just broke, and that graph on W7 showed me problem between router and internet (Authertication problem, troubleshooter says to restart router any try again, no help). Later, I took D-Link DAP1360 wireless extender that i had at home in hope to fix this issue. I put it into office (nr. 1) and began to setup it. I made it work in repeater mode (later tried AP mode, still same issue) and on survey it showed my internet connection (from TP-Link) and 54% signal strength, i choose that, wrote in his password and setup told me that everything is working fine. 4 LED's are on (when i plugged out LAN cable 3 are bright green) then i turned on my laptops WiFi and got 5/5 signal (obviously because i was 1 meter away from extender) But no, still same "authentication problem". Then i tried resetting (not restart) and put it in front of office (nr. 2) Survey showed 81% signal strength but no, still same problem. and on exactly same spot at same time i put my laptop and I can connect to WiFi. after that I went to his neighbours and when i just plugged in extender, it started working (obviously, because i set up it before) sadly, its not strong enough (or stronger than TP-link's signal, because they work under same SSID) to be working in his office from there.
And now, I have no idea what could cause it NOT to work, and as I stated up there, PRIZE are cookies! (trust me, his cookies are one of the best I've ever had, totally worth your time to help me )
Sorry if I made any grammatical mistakes, English is not my native language. If you need clarification or more information about anything, please post it here!
*Local pickup only
picture (I know, MS Paint master!):http://img40.imageshack.us/img40/8648/picgn.png