"Fideli Certa Merces" is also the motto/on the coat of arms for the Parker family:
http://parkers-history.limewebs.com/Parker_crest.html
and it "Also appears as the Crest/Coat of Arms Motto for the Parker, Shapleigh, and Bottomley families"-
http://en.wiktionary.org/wiki/fideli_certa_merces
Of course, it's also what's listed on HONORABLE DISCHARGE certificates for the Marine Corps (see above wikitionary link) & combined with the
"Semper Fi" tattoo on the other arm, I'm more inclined to think the tattoo has more to do with the USMC than this lady possibly being a Parker, Sharpleigh, or Bottomley.
Perhaps she was honorably discharged from the Marine Corps?