This is what I could imagine, a bit more than your friend's budget, but the system is sufficiently powerful with some room left for possible later upgrade. (btw I wont recommend going for branded computer in low budget case)
Component;Make;Cost
CPU;Intel-E4600;5,400
Motherboard;Intel DG31PR;3,500
RAM;1GB -800;1,000
HDD;Seagate SATA 320 GB 7200;3,000
Monitor;"LG/Samsung 17""";5,500
Keyboard + Mouse;Logitec-Set-Optical-Black;750
Cabinet + PS;Any good combination;1,500
Optical Drive;ASUS LS 20x DVD Writer;1,500
UPS;600 VA;1,500
Total Cost;;23,650