You could well be right about what is going on here. Liver enzymes COULD signify liver metastasis (spread of colon cancer to the liver). The bloating could signify ascites, fluid collection in the abdomen.
You don't really say how much of a conversation you have had with her about what you fear she fears. Depending on how well you know her, you might want to lay this on the table with her. You might try something like, "Mary, I hear you say you have rectal bleeding and I notice the changes in your body--your swollen belly and your tiredness. What does your doctor have to say about all this?" Count to 15 and wait for this answer. She may tell you she already knows she has cancer. She may tell you she doesn't want to see the darn doctor. Then you can ask her what is the worst thing he might tell her. Count to 15 (silently) and wait for this answer. If she trusts you, she will tell you her worst fears or at least some of them.
She may fear dying an interventional death.
She may fear death.
She may fear cancer diagnosis.
She may be fearful of being asked to change her lifestyle.
She may fear pain.
She may fear having to go into a home and loss of her home.
Wait and hear it.
Ask her what she sees as her options. Then let her work through her options: Evaluation and treatment, evaluation and palliation (supportive care).
Untreated cancer (assuming it is cancer):
She could bleed out (bleed to death) one day from the site of bleeding.
She could become so anemic and so weak that she faints and hits her head (sequelae of head injury) OR she is so weak that she cannot take care of herself (cannot eat, cannot perform self-care like bathing and toileting and she comes into the "system" that way. (sounds like she's going in this direction, but things could worsen.)
She obstructs, that is the cancer blocks her colon and she cannot poop, she becomes painfully distended and, in the worse case scenario, she vomits feces (poop). If the colon, under pressure, bursts or leaks, she is said to have a septic abdomen, that is the normally sterile abdominal cavity becomes infected with bacteria. This is very serious.
[the latter two possibilities are probably more likely]
Some docs regard the act of obstructing and vomiting fecal material as so repugnant (and it is, for the patient, a terrible thing) that it becomes an indication for surgery even when you cannot save someones life.
EARLY detection of colon cancer makes a huge difference and early colon cancer can be very treatable. Liver enzyme elevation (that she and her son are aware of) suggests a chance of mets and this is not early colon cancer; it may be cancer with metastasis. Be aware that heart failure can cause some mild liver enzyme derangements too but there are a host of reasons why someone might have liver enzyme changes.
Obviously being in charge of her decisions is pretty important for this lady but the time may come when the situation will overtake her.
Palliation means supportive care but not care aimed at curing the cancer. Palliation in a cancer with mets case MIGHT include: draining some ascites to make breathing more comfortable, a colostomy surgery to divert the colon past a non-resectable tumor to prevent obstruction (and this might relieve the diarrhea and the bleeding which can lead to weakness and anemia); supportive medical therapy such as pain control if it is an issue.
Her son needs to know exactly what her wishes are and he needs to have her sign a durable power of attorney and living will to protect her from the over enthusiastic doctor (if she is willing). Knowing what someone wants is accomplished through long conversations. Your local department of aging might have info on living wills and POA's. Don't rush her on this, but her best shot at having control over what happens to her next is making sure that the people that care about her KNOW what her wishes are.
As you can see, this is a complex issue and I have not said a tenth of what could be said.
Don't expect to accomplish "everything" here in one or two visits. Working through this stuff can take a long time but can be abruptly accelerated by what I call cornerstone events, such as she falls and hits her head or she obstructs.
Good luck. Your obviously care for this lady tenderly.