Hi there. I had similar surgery but they replaced the part of the tongue they removed with a bit of my arm, it looks very close to normal and functions that way too. The things you need to know - did they get clear margins? When they remove a tumor they're looking to see if they got it all - clear margins is an area around the outside of the tumor that has normal cells. If the margins aren't clear the would likely have to go back in and take more... You want to know if there is any perineural involvement - did the tumor grow into a nerve - this is an indicator if further treatment. You want to know the differentiation. Well differentiated - most like your own cells - generally fairly slow growing, moderately differentiated - a bit more aggressive, less like your own cells, poorly differentiated - very aggressive and least like your normal cells. This is also an indicator of more treatment - though with oral tongue cancer since it has a tendency to stick around I would push for aggressive treatment regardless.
Lymph nodes? They only removed one? If that's the case I would change drs ASAP. the minimum I've seen removed was 18 - with the average being 30-40 - this is also a good indicator of further treatment required. If one or more comes back positive they should and will likely push for further treatment. Or if there's ECE (extracapular extension) of the node where a bit of cancer has moved outside the node into the tissue of the neck this is also indicative of a need for further treatment..

Further treatment is generally radiation and chemo. Not fun but doable.

I have no idea what a 50-50 on a PET Means- I can tell you a pet uses a sugar contrast substance - your body uses sugar for many things... Healing.... During times of illness - and tumors are sugar hogs. So if something lights up on a PET - it could be cancer - inflammation, healing, or an infection. did they give him a PET after the surgery??? I had a CT scan after surgery - and prior to radiation (this was after my biopsy results were back. They did a chest CT - (this cancer can move into your lungs) - and a head CT to plot where to aim the radiation. (PET is similar to an MRI and CT, though apparently more accurate - but also notorious for false positives)
So maybe the PET was used to determine whether they should give more treatment (though this soon after surgery I'm not sure if I would trust it.) Ultimately the biopsy results should let you know for sure.
I'm assuming this is his oral tongue we are talking about. (the tongue in the mouth that you can see) - your tongue actually runs all the way down the back of your throat and often BOT cancer - base of tongue - has a different cause)
I would like to stress three things to you - if he's not at a CCC - he should be - regular drs are great but doctors at a comprehensive cancer center - are the best and see this cancer all the time. They would be best able to tell you what has to be done and you would likely get the optimal outcome. (though with cancer as you know there are no guarantees)
If they suggest radiation based on his biopsy - don't hum and haw about it. Do it - they're giving you the best chance to beat the cancer - its definitely not easy - radiation is directed at certain areas where they think stray cancer cells can be hiding - tumors generally show up on imaging devices (PET etc...) but cancer cells do not. And chemo is systemic so it's deigned to clean up any cells that may have moved outside of the radiation area (generally the head and neck)
Finally educate yourself. A well informed patient or caregiver - is most important - knowledge is power. Your mom being primary caregiver may not understand or take in much of what is being told to her (older people tend to just trust the drs, and let them handle it - that may be fine if you're at a top notch facility, with a top notch dr. But even then being in the know can't hurt because even the best make mistakes - )

Hopefully they caught this early. If they did then hopefully that's the end of it but tell your mom, dad, to keep an eye on his neck for swollen lymph nodes, or a rash of some sort, and have himself checked very regularly - take care and good luck!