Without further information it is very difficult for me to provide the best advice but I will certainly try.
Please reply when you can and we can continue the conversation.
If the initial lump was not removed entirely and was confirmed as a cancer, we would be highly concerned this was a recurrence of a mammary cancer.
If she was neutered later in life, or has not been neutered, this also makes cancer more likely.
A mammary strip is advised when more than one gland is affected, to minimise the risk of requiring another surgery in the future.
At 12, healthy Chihuahuas could potentially have many years of good quality life left as many live well into their teens. The smaller a dog, the longer the life span (in general).
Typically, removing growths from the outside of a body is not an especially taxing or difficult surgery for a dog to recover from. So, I would be leaning towards going ahead with the surgery in this case.