In need of help saving a mightily awesome, old, massive fir tree!?

OK, so in my garden I have this fir tree. It's been there since before I was born, and it's around 70ft tall. It's awesome. Problem is, the roots are starting to get dangerously close to a sewage pipe that runs under my lawn, and my parents have been told that the tree should be cut down, or we're gonna have some expensive sewage repairs/modifications to pay for in a few years time. Now, I've been looking online and I've found some ideas that could help (the one about inserting a pipe filled with hot water and copper sulphate (I think it was) into the trouble area sounded like the most promising), but I was wondering if anyone had any other ideas. We can't cut the roots because many of them will run deep below the surface and require us to dig up the whole garden, only to have them grow back in a few years time. PLEASE HELP ME SAVE THE TREE!!!