She is going to have green tree frogs, not grays, so they wouldn't mind a little more warmth than 68. I don't think that those temps would hurt them either, but I have no personal experience with american green tree frogs. They do come from further south in the US though, so they'd probably prefer it to be a touch warmer. I'm not sure how effective a heat mat would be behind a cork background though. I use a CHE with a dimmer and a thermostat on my red eye tank, but that might be overkill for what you would need.