Hi. Foot swelling at this age of 87 can have many reasons. Anemia , hypoproteinemia , kidney and cardiac disorders being he general cause. If there is no redness , pain locally it may not be due to infection which is worsened by uncontrolled diabetes. Elevate the legs above heart level in lying position. See Doctor's advise if there is redness / pain/ unequal swellings ( as in this case) / fever / or high blood sugar.

Thanks for query.as you said your mother is diabetic and its metformin I think .she undergoes blood transfusion frequently. the reason for pedal edema (swollen legs) might vary in a large degree.but here u have a evident cause that is fluid overload due to blood transfusions.as u mention she is taking water pills I assume lasix and still she could not makeup.ask her to put her legs at a level up than rest of the body(leg end elevation).most of the day and particularly while sleeping this should do.if the problem is persisting we should think of other causes like kidney related and should see a nephrologist.regards