You can unlock it by first powering your tablet off. After it is turned off then now press and hold down the volume button and turn on your tablet at the same time. You can now release the buttons once the tablet enters into the safe mode. DO the factory resetting.If it still didn`t work out for you Consult a tablet store for more details. This usually a force Safe Mode boot is from a bad app or hardware hickup,you can try removing the sd card and see if you can boot it clean. Another option is to try booting it into the recovery mode and clear your cache.Please let me know how it goes and please also remember to rate my service before you go no cast for that friend.
Have a great weekend.