Open the cheat console with CTRL SHIFT C then type testingcheats true to enable cheats.
Type modifyrelationship YourSimName OtherSimName 100 Friendship_Main to set the friendship level.
You can lower the value by typing a negative number or using shownotifications as false to hide prompts.
This cheat is great for storytelling or advancing relationships fast.
Remember to disable testingcheats afterwards to prevent further unintended edits.