How are they arriving in Salzburg? Car or train? Traffic is terrible at times
Panorama has a big bus tour that leaves Mirabell Garden, not far from the train station, at 2 pm. I have done that tour twice and liked it. They should be able to make that without trouble if arriving at noon. https://www.panoramatours.com/en/salzburg/tour/original-sound-of-music-tour-tour-1a-28/
A lot of people on the forum report liking Bob's Tours because they are on vans and have fewer people. But the afternoon tour starts at 1:30 and the pick up is across the river that divides Salzburg and on the opposite side of the Old Town from the train station, so I think that would be tougher to make than the Panorama tour. https://www.bobstours.com/som.php
As for a day trip to Hallstatt, that would not be worth it to me. I will allow others to add their two cents. I don't know where else they are going, but I would recommend Königssee toward Berchtesgaden as an alternative. Huge lake. Nice hike on the other side of the lake to a smaller lake and on to a gorgeous alpine meadow and then to a high waterfall. It's gorgeous. Depending on when they are there in May, there will be cows on/around the trail toward its end!
Depending on where they are headed or coming from, Halstatt may be an option along the way.