Hello from JustAnswer.
There are several possibilities to consider when there is badly swollen legs. It is good that they have excluded DVT. The blood tests may have also included tests for certain systemic illnesses that can cause swelling, such as liver or kidney dysfunction or hypothyroidism. It also would be appropriate to consider whether the swelling could be related to heart dysfunction. This can be assessed by physical examination, but an ultrasound of the heart would be more precise in assessing heart function.
If the evaluation is normal, the most common cause of swelling is from chronic venous insufficiency. When someone that is overweight develops swelling, it is often because of an exacerbation of chronic venous insufficiency. She may not have had swelling when she was heavier in the past because the veins were in better shape previously.
If she has not yet had an evaluation for the various conditions above, it would be appropriate to perform that evaluation. If the evaluation has been done, there are a couple of home interventions that can be done to try to help ease the swelling. It can help to use medical compression stockings, such as TED stockings. Regular walking may also help by massaging the veins. There also is a supplement, horse chestnut, that may be able to help improve venous tone and ease the swelling.
If I can provide any additional information, please let me know.
Please let me know if I can provide any further assistance.