。
She'd grown thin from worrying about him.
Literal
She [topic-は] him [genitive-の] thing [subject-が] worry [because-で] ended-up-thin.
The で here marks cause — 'because of, due to.' 心配で (literally 'with worry') = 'out of worry, from worrying.' The pattern ~が心配で~ frames the worry as the reason for the next clause. 痩せてしまう combines 痩せる ('lose weight, become thin') with the regretful auxiliary ~てしまう — 'ended up losing weight,' with the ~てしまう adding the rueful 'unfortunately' or 'noticeable change' nuance. Together: a small story of worry physically wearing her down.