This same unfortunate situation recently happened on a DL flight we were on about 6 weeks ago from ATL to LGW. The female passenger, about 75 years old and travelling on her own, had a heart attack a couple of rows behind us and the crew could not revive her with CPR. We were very close to Long Island so we diverted to BOS and they removed her from the front of economy when we landed at BOS. Fortunately the flight was not very full so there was no issue of cabin space when performing CPR on her. The crew were very professional and did all they could for her, but no avail.
The rest of the flight to LGW from BOS was very subdued, and the crew who performed CPR on her were emotionally drained for the rest of the flight. We gave credit to the crew for their efforts and professionalism in a very difficult situation.