CPU failure. Maybe he used it on a cloth surface and it overheated due to the vents being plugged up or you gat unlucky. It is not too expensive to buy one and to have dell do it you shouldn't have to pay more than 200-300 bucks, maybe less. The price is a guess so take it with a grain of salt.
Error is weird one - it was beeping for 2-3 days then start working. And after few days of no issues it got again into the same situation with blank screen and 7 beeps.
If CPU failed will it behave this way?
i had same issue with my dell, saw repair service on ebay. the seller id is megalab. i was sceptical first as he didnt have a lot of feedbacks butt gave it a chance and paid only $99. Vuala, my dell is alive again. try to search on ebay http://www.ebay.com/itm/7-BEEPS-Dell-Inspiron-M5010-M50...