Yes. 1) A coworker, unlike a health care provider, is not restricted from answering questions about someone's health, etc.; 2) an employer may conduct some basic inquiry into FMLA leave (or other absences from work), to see if it's legitimate--especially if anything occurred or was said that caused suspicion; 3) generally, any one can ask any one else anything--sometimes the other person legally can't answer (e.g. some health care workers about patients; attorneys about their clients) and other times they choose to not answer, but the question is not illegal.