Is the flower color different from the original (what he had last year)? It could be the rootstock that is flowering--the original desirable rose may have died. Go to helpmefind.com/roses and look up Dr. Huey and Rosa multiflora. These are common rootstocks and your neighbor's flowers might be one of them.

(Assuming your neighbor's roses are grafted onto a rootstock, rather than being own-root, or non-grafted.)

If the flowers are the same as last years, just very small, it could be due to weather, or a need for fertilizer...

he could have cut off too much at the bottom and what is left is a sport of the original flower. he might have pruned too much on the actual flower and killed it.
That is my only thoughts, hope it helps.

If the new blooms are the same color and form as the old blooms, then weather or fertilizing or watering or pruning could be the culprit. Some roses do not respond well to a hard pruning (I know, I have one that is sulking to high-heaven now), but bounce back well the following year. Could have been the time of year that he pruned too, if it was late (like late April) then the roses want to put out buds now, and those may be smaller than usual. And if you had any of those wacky cold snaps in March and April, that impacts blooms size too. Lack of good watering and fertilizing can also impact blooms size.

If the color is different, then it is what hoovb said, he's lost the original rose (and this didn't just happen with this pruning), and he has rootstock now. Dr. Huey is looking particularly lovely in a neighbor's yars at the moment.