Put the tree outside now, for life. It has no business indoors, let alone in an east window, especially in the UK. Your friend may have watered too much or not enough - can't tell from here obviously - but it won't get better where it is and not knowing anything else about it, what it's in, etc. can't say more now. For the first week or so however, if possible, move it out of direct sun during midday for a few hours (again, not knowing your day to day weather, it's hard to say).

Agreed: outdoors year round. Repot in bonsai soil soon, give a good trim (roots & top) & place in semi-shade to recuperate. Privets are tough -- it'll probably bounce back.