Several of the RAF online resources show 57 Sqn at Feltwell with Wellingtons (the Sqn code was DX) until Sep 1942 when they re-equipped with Lancasters. So - it looks like he was part of a Wellington crew. A bit more searching showed that on the night of 25/26 March 1942 Wellingtons were involved in 2 raids. One , on St Nazaire, lost 1 Wellington. It is unlikely that is your aircraft. The other, a force of 242 mixed aircraft included 192 Wellingtons raided Essen. Three were lost - one of which was (I think) Z1321 which crashed near Vlaardingen in the Netherlands, hit by AA fire (Source: RNZAF Wellington website). I could find no more but if you can find someone who has a copy of the Bomber Command War Diaries, or the 57 Sqn Operational Record Book (ORB) you might be able to find out more.
I expect you've noticed, from the Commonwealth War Graves site, that he is buried in a collective grave with 2 others. Their names are Freeman (Observer) and Hill (WOp/AG). No sign of any other crew so their bodies may never have been found or they may have been repatriated after the War.
I wish you the very best of luck with your search on behalf of this lady - well done you for helping out!