How old is this computer?
The symptoms you describe could be the result of an overheating processor. If your friend is happy about opening the case get him/her to check that the processor fan is running when the machine is switched on.
While the case is open - and the computer isn't running - is an an ideal time to remove any accumulated dust and fluff, and to carry out a check on all the component connections - leads and cards, etc. Check the connection between the graphics card and the monitor, replacing the cable for good luck.
Try all that, and let us know what happens - or what doesn't happen.