silver birch problem

Mums had a silver birch tree for years, and last year I had to take drastic action to top it, so chopped main stem sealed it, All seemed fine this spring it was a little slow to come to leaf, but once it got going it was the most beautiful it had been in years, Few weeks ago we had the gas people next door dig up their garden and the mums tree was only a 3 foot away from the area they were digging and in my mission to try and find old concrete fence post holes,( on going battle over boundry line, another story ) discovered the roots which i assume is from the birch needless to say I damaged one or two digging......

Now last couple of weeks mums noticed leaves going yellow from the centre then out and on close inspection its looks what I can only describe as extensive rust spot but yellow not orange. No doubt the gas people would of cut into some roots or even cut off as they would of had no concern over a tree, Has it got a disease or is it slowly dieing ? it there a way to save this tree?