Aggressive Behavior towards Owner

I need some help from some of you cat experts....My best friend has a male cat named Flash....a long-haired black and white ~2 yr old neutered cat. An only cat.
The past month or so she has had some trouble with him. First, he started peeing outside of the litter box, all around the house. This was accompanied by some aggressive behavior towards her, hissing, biting and scratching. She took him to the vet and the vet put him on antibiotics in case of bladder infection, urinary tract infection, etc, I'm assuming. They couldn't get a urine sample I guess? So per Cheryl, my friend, the vet put him on meds.
Well, his aggressive behavior seems to be increasing. He attacks her when she walks around the house, jumping out at her. Hissing, ears flat, tail flapping when she even walks in the room. The other day,she said he was on the counter and he leaped onto her and bite her and she noticed drops of urine on the countertop.....I haven't been able to be much help to her because my knowledge of cat behavior is very limited. I thought maybe he was in pain...He's young, has all his shots, is a healthy weight and sustaining it during this time, appetite remains the same. I didn't ask about his water intake? He is an indoor cat, never goes outside unsupervised, if at all.
Any ideas? Or any websites I can check? I checked one, vetinfo.com and it wasn't very helpful....
She is at her wits end! They were very close and she says now they get in a "fight" every morning and evening!

Thanks!!!

Has she changed her perfume? Shampoo? Laundry detergent? Or has she done anything different, like have a new boyfriend or roommate? It seems like this cat is displaying EXTREME dipleasure at something she is doing.. so it may be traced back to her, not the cat. I don't know.. I'm not too good with cat behaviours like this but that's the first thing I thought of. I'm sure you will get other responses that have more info.

Good luck! :)

Jennifert...check out the thread a few below this titled <Aggressive, Strangely so.." as your friend's cat seems to rival my friend's cat...the people had some good suggestions.

Thanks for your help and advice! I shared your thoughts with Cheryl and she is racking her brain trying to think if she has changed anything.....So far, she can't think of anything....But it may be something simple that she is overlooking....
Thanks again! I'll keep everyone posted on Flash and his progress.

Some cats are more territorial than others. They show it more when they get to a certain age. The peeing outside the litter box is, most likely, Flash marking his territory. Your friend should try and find some way to state her authority. Let him know that he isn't in charge. Our female cat, Tiga used to attack everyone and everything in our house. We resorted to spraying her with a water bottle whenever she attacked someone or we caught her peeing on something. She knows not to do it now and is a great companion. There's no guarantee that this works with every cat, but it worked for us. Good Luck!
-Alpha Wolf (Shay)

Well, thanks for the advice everyone. Over the weekend Flash has started doing #2 in the house, diarrhea in any and every room....He has a vet appt this am at 9:15 so hopefully he will be ok....

I think her cat is most definately trying to tell her something! Did the vet check him thorughly? I'm anxious to hear the result! It sounds like a bladder infection + ??

Good luck! :)