Sorry to hear it STE.

With bowel cancer it is often the secondaries that kill (cancer in other organs) - and people don't always know immediately whether they have them. Sometimes bowel cancer is operable, sometimes it is treatable with radio/chemo. In other words, there are many prognoses.

In terms of behaviour, you know this person best so know how to respond to them. In general I would say be your usual self and be cheery and entertaining. Often the person themselves will feel that they have to cheer up friends and family around them and this can be quite an effort.

However, also be sensitive to the fact that certain things might change and once they start having treatment they will not be quite the same person. These treatments are extremely powerful and cause all sorts of symptoms and difficulties.

You can help by helping the people around her too.

ETA: much the same as Claudio's advice really

ETAA: with hugs etc and telling people you love them, it may seem weird but I think it's more likely than not that this will be deeply appreciated :)