My friends computer was hacked because he clicked on spam, and since we use the same internet line IP address, I was wondering if the hacker can access other computers in the house that don't click on spam and weird websites, just through the internet line IP address?

Yes, easily.

Of course, clicking on spam probably just gave him a virus. Though, these can still infect other computers on the network.

So basically, yes, easily.

Are you saying if I was to use a brand new computer and use a hardwired cord to connect to the same internet where the other computer has been hacked, the hacker can hack the new computer, if so how?

Are you the op? You don’t have to make a new account to post every time.

There are Network viruses that self-replicate. Anything an automated virus can do a hacker can do, because viruses are written by hackers and reproduce the skills the hackers use.

If your device is on the network, it’s trivial to discover it. From there, a quick port scan can show what services you may have running. After that, just looking at common vulnerabilities will usually be enough to get them in, since few people keep their devices up to date. This assume the attacker doesn’t want to do a man in the middle attack, which is much simpler.

Yeah I misplaced password

So why is it advised to use a separate computer for say poker only if it doesn't really make that much a difference?

If you keep your other computer on all the time, yeah, might not matter in some cases.

However, very few viruses are going to be directly controlled by a hacker unless they know you’re a whale/high priority target.

Also, network viruses aren’t necessarily as common as run of the mill viruses.

If you don't know the answer to this, then the answer is yes. (that's only partially facetious)

For a completely non-facetious answer, some more information is necessary.
For starters, how is your network physically configured? Based on post #3, it sounds like you are saying that you are connecting to a shared internet device (most likely a vendor-provided router) via an ethernet cable, and not by a wireless connection, yes?

Also, just saying "he was hacked" is near-useless. There are many different forms of malware that have many varying objectives, with wholly different methods of propagation, etc,etc. So any info on what kind of virus your buddy has, and how it was diagnosed would help.

It's really unlikely that your computer will end up hacked because another computer on your network was hacked. I would only be concerned if I thought I was being targeted, like you are publicly known to have lots of poker money or Bitcoin. Or if I identified the particular malware on their computer as one that can spread via network, like ransomware.