Fiv cat

My fiv cat got out and had a fight with the neighbours cat. As far as i know their cat just sustained a couple of scratches to the ears. They dont know our cat has FIV. 3 weeks later, their cat suddenly has fluid on the lungs and is on antibitoics for a chest infection. Im terrified it might have been caused by our cat in case it gave their cat fiv. Is this likely?

Oh my goodness!  What a horrible position to be in.  It is not common for FIV to cause symptoms so quickly - usually it sits dormant in the body for a while first, although it's not impossible (particularly if the other cat was immunocompromised for another reason at the time of the tussle) - and if they have been fighting once, they might have been fighting before unbeknown to you.  It is generally reccommended to keep cats with FIV indoors, and now you can understand why.  As you know, FIV is spread by fighting.  I wonder what the 'chest infection' was, and the diagnosis for the fluid on the lungs?  I see that you didn't ask me for advice about the moral dilemma, but for what it's worth, I feel that honest and open discussions are generally a good policy.  If both cats do turn out to have FIV, it may still be unclear which was infected first, and it sounds to be unclear at the moment what is causing the other cats' signs.