I was fighting with my friend

[keannu]

I was fighting with my friend for a long time, but that friend came to celebrate our wedding.
=> I fought my friend for a long time, but that friend came to celebrate my wedding.

I think the first sentence should be corrected to the second one.
I think "fight sb" is different from "fight with sb" - the latter means both you and sb fight a third party, right?

 

[slevlife]

No. From context, “fighting with my friend” means my friend and I were in a (probably significant) argument/dispute/quarrel.

I would not change it to the second sentence. Dropping “with” makes the fight sound more likely to be physical.