We all want to stretch the capabilities of our team members. At the same time, think about minimizing failure when someone has a "stretch" goal with high risk attached. Any high risk goal can damage your reputation, your staff member, and even the organization.
Manage risks by discussing the actions your person plans to implement. Then, monitor the results and agree on frequent reviews to catch anything that needs adjusting before things get "off track." Stretch goals are great confidence builders for everyone involved; they also require more follow up conversations than low risk goals.
Speaking of low or lower risk activities: it usually works well to have the person you're coaching act first, then report back and discuss how things went and what was learned.
Important point: Solid coaching agreements include mutual responsibility. Show loyalty to your staff even if something goes wrong, then help them pinpoint lessons learned for the future.