I say you deserve better than to be a "text buddy".
Let her know you like her but your don't think it's appropriate to continue to have lunch, text, etc... If it did become something more, you don't want to start something off on a bad foot...and I am sure you wouldn't want your girlfriend to do that to you.
If she is not happy in her relationship and is truly interested in you, she needs to put on her big girl panties let him know and not play games w/ either of you. Until then, I say take the high road.
me: 30 H:30 tgthr:7 m:4 no kids 5 counseling sessions initiated by H as a LR: Oct 2012 long distance marriage b/c of work since Nov 2012 official BD: July 2013 nothing filed 1/1/14 I dropped the rope