Support or immaturity/whipped?

Hi everyone,
Wanted your opinion on something. I've seen this situation time after time again. A specific eposide I'll share is how earlier this year before the pandemic shutdowns, I went out to eat with my FH, his brothet (FBIL) and his gf. The service was disappointing. The server dropped a plate of things and milk splattered everywhere on our clothes, in our food snd it spilled on the clothes of two other tables. The server refused to apologize to us. Mind you, I'm allergic to dairy. When we were ready for the check we continued to make eye contact with her and she would not look at us foe 10+ minutes. I ended up tipping her, but not my typical percentage which is 20%. I also wrote a note explaining why I tipped lower and how I was disatisfied with her service. Which my FH saw what I was writing. I didn't feel it was necessary to write a negative review nor conplain to the manager. I did not want to embarrass her.

My FH abd I left first while his brother & gf stayed behind.
The next month, when I saw my FBIL's gf, she looked me up & down with dagger eyes and wore a shiet that read "got milk?" that had a pic of a glass of milk lol! She ignored me when I greeted her and so did her BIL. I seriously had no idea why and was confused.
The following month during when quarantine began my FH told me that apparantly the server read the note and was upset and claimed to my FBIL & his gf that I wrote curse words in the note!!!! Which is NOT true. FH asked them if the server showed them the note so that they saw it with their own eyes. And FBIL replied, "no, she threw it away..."
FIRST OFF, when a customer does indeed write something crazy, a server would not immediately throw it away they would definitely take pics!
I immediately called my FBIL and put him on speaker and asked him why didn't he talk to me about it instead of being petty and rude and he replies "oh my gf wouldn't let me." Like what?! I then said "umm she did not put a gun up to your head. That was YOUR decision to not confront me to ask me if I in fact wrote some crazy s- word. And it's very disappointing how immature and shady you acted because over the past 5 years I've been with your brother, I have spent THOUSANDS of dollars and hundereds of hours on/with your child who lives with your guys' parents and you have NEVER thanked me. He responds, "I SUPPORT MY GF. IF SHE DOESN'T LIKE SOMEONE THEN I DONT LIKE THEM EITHER."
One month later, his gf breaks up with him. Then in July, she marries someone else!


Now, FBIL tries to act all friendly with me but I'm not having it. I greet him with a hello and a handshake and will only discuss the weather with him. I make sure to not say any negative things about him to my FH because I know it's unnecessary.

Umm is it "supporting" to be openly disrespectul to someone your significant other doesn't like? Even If the reasoning doesn't involve bodily harm, sexual assult, grand theft, something serious, etc.?! Would love to know your thoughts.