A Product Owner wants advice from the Scrum Master about estimating work in Scrum.

According to the Scrum Guide, estimates are made by the people doing the work, which is the Development Team. The Development Team is responsible for all estimates in the Product Backlog and the Sprint Backlog. The other options are not valid guidelines for estimating work in Scrum, as they are either too prescriptive (such as requiring story points or relative units), incorrect (such as forbidding estimating or having the Product Owner make estimates), or unnecessary (such as checking estimates with the Development Team).