Unless planted as a large specimen, a comparatively short time ago at 15' tall this small-growing tree is likely to be well past the time where the grass might be interfering with its establishment and development, making reduction of grass area unneeded now. In fact, the time may be here when it is desirable to try to get more grass to grow under it, so that the lawn comes right up to the trunk. This looks better than when a tree is coming out of a hole in a lawn. Trees and grass are enemies, some allowance for this is often necessary to maintain a balance resulting in a pleasant scene. When trees are new and small you want to keep the grass back from the trees. Later, to maintain an attractive effect you may have to limb the trees up so that they do not kill part of the lawn by blocking out the sun or deflecting precipitation.

It was suggested "so that the lawn comes right up to the trunk."

Doing so makes it much easier for one to cause mower-blight and/or string weeder damage.

Keep the grass back at least 20 inch radius. Then as the tree grows larger & becomes more dense, you may want to expand the circle more, if only because the now shaded grass is thin, spindly & sparse.

Anytime you have anything verging on grass you have to learn how to trim next to or around it without damage.

Possible if you're very careful & particular. A fair number of "landscape care" companies aren't so much.

Hawthorns are tough trees. Removing a little grass around the trunk won't help one bit other than to possibly lessen the chance the trunk will get hit with a lawnmower or string trimmer. Hawthorns are typically used for street trees in the parking strip where conditions aren't noted for being good.

Personally, I wouldn't have one in my yard, or in the parking strip. All too common, and with a host of problems. Thorns, soot mold, and a branching pattern that resembles a brush pile. Pruning one is a nightmare and so is the disposal of the branches. Good firewood though.

Different kinds have different attributes. 'Paul's Scarlet' (or another kind sold in its place) is the only one commonly offered here that has a chronic, predictable health problem (hawthorn leaf spot). Washington hawthorn can be quite nice, with an attractive branch pattern, pretty leaves that turn multiple colors in fall, and abundant crops of small red fruits.

I will enjoy the Washington Hawthorn in someone else's garden or Arboretum.
One form with the normal white blossoms has escaped here locally and is now inhabiting fields once grazed by horses and cows. Nice for wildlife though.

Your friend may be parroting a common ideal scenerio promooted by arborists. Meaning in an ideal world, trees would have mulch out to the dripline. But in the real world where that is not always practical or enjoyable, it can be minor and inconsequential sometimes.

You have cleared enough.

Also, removing grass sometimes damages surface roots. Roots can be cut, but why damage some, if the grass is back far enough.

The locally weedy common hawthorn (Crataegus monogyna) is not a form of Washington hawthorn, but rather a completely different species - and does not look that much like it - save for also being a hawthorn.

We also have native species of hawthorns growing in wetlands here. Again, these are not particularly similar to Washington hawthorn, as far as it goes.

Did your friend say why you need to remove more grass?

You probably have enough, unless there's a problem with the tree. But a 15' tall tree is generally going to hold its own against a lawn, usually the lawn is the loser in these battles.

If the tree is having problems it may be that 'worst clay imaginable' that it's growing in, not the grass.

The weedy hawthorn is a major invader of grassy wetlands and wet meadows around here. Not a bit bothered at growing up through grass.

It's good to know there is a 'Good' Hawthorn out there.
I had my doubts.

The two that are common and conspicuous in local plantings at this time of year are Washington hawthorn and Lavalle hawthorn. Fruits are also still present on common hawthorn but these are no longer red. Some Washington hawthorns west of I-5, in WA, not far south of The Plant Farm at Smokey Point are still solid with numbers of tiny bright red fruits. Being half Mexican hawthorn, the Lavalle hawthorn produces instead much smaller numbers of larger fruits, often appearing orange from a distance. The native black hawthorns produce black fruits that are never spectacular.