At LAS, Concourses D/E and Concourses A/B/C are not connected airside, and at Terminal 1, the security lines for C and D are separate (the security line for Concourse D PreCheck/elite/F is adjacent to the security line for Concourse C, but they are separate).

At Terminal 1, the tram to D gates is separated from the tram to C gates by a locked door. Based on FT reports, it is manned by TSA or possibly police. FT'ers have had varied success in getting the people manning the doors to let them through. One FT'er reported a year ago that he got through without even having his BP examined (http://www.flyertalk.com/forum/las-v...-amex-las.html), but there are reports of denials (e.g., multiple posts in http://www.flyertalk.com/forum/south...g-d-b-las.html and http://www.flyertalk.com/forum/las-v...rs-thread.html).

To guarantee entry into A/B/C, buy a refundable ticket on an airline operating out of A/B/C. After you're done meeting up, then you can try getting back over to Concurse D through the locked door at the Terminal 1 tram station, but leaving enough time to exit and re-clear security in case you are denied.

The other possibility is getting a gate pass, which is up to check-in agent discretion, but this seems logistically unlikely since DL probably won't issue a gate pass to A/B/C - and obviously you won't be with your relative landside so you won't have any reason to get a gate pass from the airline your relative is flying.

Unfortunately LAS is pretty sparse for sit-down dining options and even the chains can be lousy (CPK for example does not even serve the same food that their normal chain locations serve). I also have no knowledge of A/B/C, so can't help you on that.