Hi, sorry for being so late getting back to you
Here is a link to Treacy's thread, I haven't really read through it again as I can't bring myself to yet, but it charts what went on... It doesn't end well as you will expect but there are details of meds etc in the thread...
When reading it just bear in mind Treacle suffered with bloat before her Lymphoma so don't panic and expect that to happen..
http://www.theguineapigforum.co.uk/threads/treacle-has-lymphoma.103647/
Treacle was treated with steroids, these shrink the lumps (hopefully) and are given by injections at intervals agreed by the vets, they will slow the disease down but not stop it. The main thing is quality of life, our vet assured us Treacy was in no pain and Treacle for the large part of her battle with the illness was her normal happy self. If she had started to be in pain, struggle we would have PTS earlier than we did.
So I would discuss Steroids with your vet and go for regular check ups to feel the lymph lumps they will appear under the chin and noticably back of piggie knees... Remember as the cancer grows they will probably get more hungry and weigh more as the lumps are heavy. Keep regular weighing, maybe a little extra amount of veg seperate to their friends (We spoilt Treacle daily with her favorites lol and would take her out to give her a little extra) and top up with CC, Oats etc... as you feel nessacary. We didn't start syringe feeding until near the end. On Steroids their apettitie increases so make sure they have extra food to compensate.
The lymphoma affected Treacles eyes and made the red and swollen our vet told us some eye drops to use to help soothe them.
The other downside of steroids is they weeken the immune system so make piggies more susecptible to infections, just keep a close eye out for them. Treacle suffered with Bladder issues and was plagued with UTI towards the end, you can't use metacam with steroid but Opiate based painkillers are available and are detailed in Treacles thread, opiates when given alongside a gut stim like Emeprid are effective and the gut stim stops them from slowing down the gut too much.
I know how painful this is to take, you will do the best for Ruby and make everyday you have with her a great day. Our last month with Treacle was very bitter sweet, she bonded so close to us having her meds and cuddles and would come over to me to be picked up, something she would never have done before.
I hope I haven't rambled too much, I'm not a wordy person... Just aim for quality of life as you guidance and it will make decisions as easy as they can in this awful situation. If you ever want to chat I will pm you my number and you can always find me on here.
Love to Ruby and a hug to you x