To the extent that you would really like to maintain the adverbial feel that “refreshingly” has in your English example, perhaps you could consider trying to fit the “de/d’une façon +adjective [rafraîchissante, in this case]” construction somewhere into the translation, such as:
Ma collègue s’est montrée franche/honnête/ouverte, de/d’une façon
rafraîchissante, lorsque je lui ai demandé son avis.
De/D’une façon rafraîchissante, ma collègue s’est montrée franche/honnête/ouverte lorsque je lui ai demandé son avis.
You’ll perhaps note that these suggestions are similar in form to @Ah alone-zee ’s good answer (and that I’ve also borrowed “s’est montrée franche” from that answer [as well as “lorsque je lui ai demandé son avis” from jlliagre’s]).
However, perhaps my suggested choice of words doesn’t imply quite as much as Ah alone-zee’s (to me, at least) that this particular colleague is usually not frank, but instead that people/colleagues in general are often less than frank.
Therefore, perhaps my choice would be more appropriate if this colleague is actually being commended for being an exception to this general rule/observation and not being indirectly/backhandedly chastised for her particular prior lack of honesty.
(cf: as an aside relating to my earlier comment above that I sometimes use “welcomingly” instead of “refreshingly,” perhaps you could use “De/D’une façon bienvenue” instead of “De/D’une façon rafraîchissante” in either of the above suggestions without changing their meaning too much)