avicii

My grandmother aged 75 has a 30 year history of RhA had a MI 6 weeks ago and was hospitalised, during which time she developed an ulcer measuring 5mm x 5mm with a sloughy base probing to the bone on the 5th digit of her right foot.

She is on the following prescription:
5mg Prednisolone b.d.
75mg Methotrexate plus Folic Acid q.w.
5mg Warfarin o.d.

She lost 7kg while in hospital and has since been eating well and has gained 1kg. she complains of an unremitting pain in her right foot and knee, which is worse around the site of the ulcer. This makes it difficult for her to mobilise. She is worried about several black patches around her nails in all her fingers and toes, including the tips.

Could this be gangrene? And from what I've been reading it seems likely shee may also have PAD. If it is gangrene is amputation the only option or can other avenues be explored? What investigations/treatments are there? Any help would be much appreciated.