your friend is right. it is best if they dont have to compete with other plants or grasses for nutrients and water. One way around this in container culture is to find a pot bigger than the one the citrus is in and place the citrus inside it. Fill the space with soil and plant annuals or whatever in that space. Essentially the plants have their own growing area and the plants dont have to compete for food and water.
mike

What Mike said. Citrus are big nitrogen demanders, and their feeder roots exist in the top 18" of soil. Not sure if your tree is in the ground or in a pot, either way, nothing under your tree but some mulch to keep the grass out (and keep the mulch away from the trunk.)

Patty S.

Regardless, nasturtiums are a plagued by a variety of pests themselves!

...aphids, cabbage looper, corn earworm, and flea beetles, just to name a few.