It resembles the plant which gives citronella oil.

These are accidentally called geraniums by most gardeners. Does it match yours?

OPs plant isn't a Pelargonium. My nearest thought would be Tanacetum parthenium, Feverfew.

That's what I thought of also. But: are the leaves too long and fern-like? I didn't check.

Thanks for the suggestions. Feverfew was my tentative guess; perhaps the late-season sprouting resulted in a compact plant. I will inform the owner; the plant will probably stay put until it blooms next year.