Sees less information on Scarlet/Violet specifically. Assumes similar mechanics as Sword/Shield. Should be able to reduce it by fainting the Pokemon or using bitter medicine. Purchases bitter medicine at Chansey Supply. (One such Chansey Supply location: Mesagoza.)
Note: Caps out at 160 Friendship normally. Only goes above that (and getting the in-battle bonuses) by doing picnic stuff like sandwiches and washing them. Avoid the EV-reducing berries too. Works the same as Sword/Shield, in that respect.
According to Serebii:
180 - 219 Friendship: 10% chance of surviving a knockout
220 - 254 Friendship: 15% chance of surviving a knockout
220 - 255 Friendship: 20% chance of healing its status
255 Friendship: 20% chance of surviving a knockout, double critical hit rate, and 10% chance of avoiding an attack.
To check your current friendship level, look for someone in Cascarrafa in the center near a fountain.
150 - 199 Friendship: "You're getting along well...but I'm sure you could get way friendlier!"
200 - 229 Friendship: "You're good friends, all right! You definitely enjoy each other's company!"
Figures the rest out from there.