Do I stay or do I go?

Hi bear91​  -  I am so sorry to hear of your diagnosis.  Tri​ has given some great advice - gather as much info as you can, to help you make the decision .... 
This bloody diagnosis mucks even more with your brain than your body - so lining up all your options and trying to make the best decision for YOU is the way to go. 

Slow down, take lots of deep breaths .... hopefully it will all pan out nicely for you xx

It almost sounds like you are hoping/planning on staying on in Australia, if not permanently, then long-term ....
 
Do you know if you are covered to have your surgery & treatment here - or will you have to pay for it?  UK does have a reciprocal medical agreement with Australia (as does NZ.)
https://www.servicesaustralia.gov.au/reciprocal-health-care-agreements-visiting-from-united-kingdom?context=22481#a1

How long have you been with your employer on the visa?  Is there any 'medical insurance' included in the job?   Are you given sick leave & holiday leave?  Do you have any up your sleeve?  Is there any way that you can use these up for your surgery/recovery (and if ongoing treatment?) so that it doesn't interfere with your visa?

Having your friend's mum backing you up near the hospital is a real bonus ... as she fully understands what you are going thru.   I bet she will almost 'become like family' too xx

If you have a mastectomy, and don't require chemo, the recovery from surgery is often 'easier' than having 'lesser' surgery (lumpectomy/partial mastectomy) tho every person's recovery is different.

If you have radiation - it will require you to be near the facility for daily visits (Mon-Fri) for as many weeks as they choose you to have it .... 

Fingers crossed, you just need the mastectomy & hormone meds xx

Take care, maybe even contact the hospital admin to see if they can give you any info as well? Wishing you all the best xx