What happens if/when somebody decides to get a friend who votes for all their posts and vice/versa? Would that be bad?
It's bad if it's dishonest.
Because we, society, have agreed that being dishonest is bad. However, if the two people legitimately like the other person's posts for whatever reason then it's not bad.
The obvious extension would be creating a second account to vote for all your posts. That's bad. It's inherently dishonest.
In either case it is ultimately pointless. These are internet points. You can't spend them and falsely inflating them will not give anyone a sense of accomplishment.
Moderators can see the voting behavior of users. Although they can't see the specific details of each vote, they have the metadata. Furthermore, if you up-vote too many posts of the same user in a short time, the system can undo it automatically under certain conditions: What is serial voting and how does it affect me?
If it's done in good faith, you should not have problems if you give a handful of votes now and then. Still, I would encourage you to upvote: not just your friend's posts but all the posts you consider useful as well.