I was wondering how there would be swelling (or edema) in the feet when I realized it must be because of the poor blood circulation and subsequent fluid shifts. Anyway, lasix (furosemide) is a diuretic that helps in reducing fluid accumulation that manifests as swelling. In the process, it reduces blood pressure. Your mother-in-law may also elevate her feet to draw water back to the upper part of the legs or to promote venous return. Be careful not to massage the swollen feet or she might develop thrombophlebitis.