[Ontario] gf got into car accident. Opposite side has a friend as a 'fake' witness. What is the best course of action?

Hi,

My GF (her dad in the car too) got into a car accident last night. She was at the intersection (green light), decided to change to the right lane, tuned on right signal, checked blind spot, saw that Car-A has stopped, and proceed to switch lane. Car-A then hit GF's car.

While CarA is parking onto the side, the driver in the car behind her, Car-B, got out. My friend then asked if Car-B driver know Car-A, she said yes. However when Car-A driver came out, she immediate said she doesn't know Car-A, (I assume it is so Car-B can be Car-A witness).

Anyways in the end my friend and Car-A decided to settle this without contacting insurance, afraid that it might raise insurance premium. They both exchanged insurance card, and car license plate number. However they did not contact police or insurance company. The plan is to meet at a mechanic shop and see how much it cost to fix the damage.

What are all the options for my friend and which one is the best?

.

additional info:

Car-A & Car-B claim it was red light, but GF know for a fact it was a green light. GF was able to change lane because the car in-front of Car-A had crossed the intersection before the hit.