Does your sister only have one rat? If so it is very important that rats have same species company.
I hope little Mr. Rattie ends up being okay, rats can live very happily for months with tumors, so it isn't an automatic death sentence. Two of my ratties had mammary lumps and I chose not to have them removed because both of my girls were over 2 years, had battled with respiratory infections for most of their lives and had damaged lungs as a result. Libertine lived a for 5 months after and remained completely happy in herself the entire time. Vertigo lived happily for 4 months before she began having problems getting around (she was born with only 1 front arm, the lump completely covered her little stump making it very difficult for her to get around) and lost her will to go on, it was heartbreaking because despite her handicap she could do anything as good as or better than the other rats, and she was the alpha.
As for neutering, I wish it were only $70! I have never heard of it being that low for rats. One of my rattie boys, Albion was having behavioural issues so I had him neutered at a cost of $150 and I was quoted at $350 by the only other exotics vet in the city. It was worth it because until that point he could not live with other rats and he was always on the verge of biting me, so he had absolutely no companionship - he was never able to live with other rats but I did develop a very close relationship with him.