Mum had DVT followed by a stenotic vein and swollen leg and foot now she has thrombophlebitis is there a lifelong treatment?

Best to. check to see if her blood is too thick - hypercoagulable. Need to remove or dissolve current clot. The underlying narrowing then can be treated - possibly a stent. She may need Coumadin (warfarin) or Xeralto for a lifetime. Once clot gone use compression hose for leg swelling. Elevate her legs when possible.
See Vascular Surgeon. I would recommend that you see a vascular surgeon for a thorough assesment for treatment options. Hypercoagulable workup indicated. Stenotic vein may be amendable to angioplasty or stent if it is in the iliac vein. Lower stenotic veins often managed with RX compression stockings & possible blood thinners. She may need lifelong blood thinner medication. She likely needs venogram & ultrasounds.