Dogs are usually spayed if they get mammary tumours, and it does reduce the risk, but the older they are the less it reduces it, and after a certain age, it doesn't really reduce the risk at all. That's what my vet said anyway. Because mammary tumours are caused by hormones.
I guess it's not usually done for guinea pigs because it's so risky for them. And also guinea pigs only have two mammary glands, so it's a lot less likely to come back. Dogs have so many mammary glands. The dog I saw in the vet kept getting tumours on the right hand side glands. I wonder with guinea pigs, if you just removed both mammary glands, wouldn't that be easier than spaying . . . I dunno.
Personally I would just get a second or even third opinion about spaying.