Hi Patrick,
Costa Rica has a good medical system, both public and private, that rivals care in the U.S. for many conditions. But I’d say as far as cancer treatment it’s not on the level of the U.S. or Canada.
There are cancer treatment centers, and I’m sure there are many hospitals and clinics that can screen as well. They’re centered in and around San Jose, the capital. My advice would be to contact some facilities in Costa Rica to ask questions about your wife’s particular case. They’ll be able to tell you exactly what they offer. All these places have English speaking reps, and they’re experienced in dealing with international/expat patients.
CIMA Hospital: http://www.hospitalsanjose.net
Clinica Biblica: http://www.clinicabiblica.com/eng/index.php
You might also check out this article from a local paper (in English) that mentions a new cancer center down here and some of the other treatment options.