Hi, you must replace that fan, with a blade broken it is now running out of balance which will also mean less air being moved. The vibration may cause damage by loosening things, connections, screws holding things in place etc.
If you are looking at replacing the entire cooling system as per post #13, then go ahead with that, though also bear in mind your friend might have a point that the CPU may now be faulty hence why it heats up too much.
Nev.