The only repercussion is you cannot find him alive later.
Convince Darla to leave, (save before you try speech check in case you fail) but before you do place one Frag Mine at their feet, it will not activate. Convince her to leave, back up, as soon as she is clear, while there are still talking, lob 1 or 2 plasma grenades, makes for a one shot kill on all of them. If Darla is still close, she goes down to and Nick has to tell her parents she is dead. Easiest way, and quite a sight. (The Frag mine is set off by the plasma nade(s) but wont activate on them as long as they are green.)