Unless the lump is preventing her from moving around so she can eat, drink, and enjoy life, there's no need to think about euthanasia yet. See what the vet says and go from there, but if he/she sees that she has recovered well, they may reconsider their earlier position. I've had two tumour removals on rats before, and they bounced beck after both surgeries, although I would suggest waiting at least 6-8 weeks between surgeries to let her fully heal internally and externally between surgeries.