If she's planting seeds she can do it right after spraying because Round-Up has no residual effects. If she’s putting in plants, technically she can plant after the spray has dried, but I like to wait at least 2 days before planting because the glyphosate needs time to pass through the weed to kill it.
Note that spraying Round-Up on bare ground is a waste of money. It has to come into contact with the “green” material to kill the plant.
There is some research that shows damage to tree bark, so be careful
Assuming it wasn't Roundup Extended Control you should be able to plant right away. But the Extended Control sticks around for several months (it's best used for killing weeds in cracks in the sidewalk/driveway and other places where you really never want anything to grow, but her hubby might not have known that when he went out to buy something.
Good knowledge to have. I went by there and what Uncle Bart used was not Extended Control. I I told them to give it to, at minimum, the weekend before putting in the annuals. Since they are retired and time constraints are not an issue they decided to wait til mid week.
Oh I hope it goes well because she is really excited about her little flower garden.