what is a good martial arts styles to defend against aikido and jiu jitzu?

ANY style that trains realistically.

and it depends on how he trains.

generally aikido schools don't have good training methodology- ie: they don't train against a fully resisting opponent, oh like most schools that don't they have a whole host of excuses so I'm not singling out aikido here- just bad training methodology.

That doesn't mean your friend doesn't train at one of the monority of schools that might- but on average they don't train realistically.

and if you don't train realistically, against a resisting opponent, you won't learn to apply them- you won't be compliant in letting him get the move, so if he didn't train against someone that was- what makes him think he can do it on you.

same with jiu jitsu- except jiu jitsu schools tend to have more schools that train realistically however there are still those that are bs.

my advice to you would be to find a good sportative art to train in- analyze the rules and find the gaps and weaknesses that are there.

because he is training jiujitsu I would suggest you take a sportative art that incorporates grappling as well- or study it seperately. however if your sole desire is to learn MA to beat your friend, then I suggest you just suck it up and find something you actually enjoy doing. martial arts is for fighting, not for making crappy home movies and interpretive dance routines (aka katas).

just piss him off by NOT bieng compliant with him when he wants to "demonstrate" something on you- if it works, hes learning, if not odds are he either isn't learning and/or is just inexperienced or he is training at a crappy school.

I don't practice Aikido but from what I've seen its a definite yes for Tae Kwan Do attacks. Most of those kicks have no control and just let loose. It would be very easy for someone (with practice) to just step back and grab the persons heel then pull it and the guy will fall. About Muay Thai though, I have much respect for that style. Its just totally monstrous. If I had to counter a low sheen kick, it would probably be by trying to be faster and kick first. I don't know what Aikido practitioners would do, but I can't believe they've never considered it.

your gunna want something fast and pretty powerful

Wing Chun is great

but it doesn't matter what martial art it will be

what you need to do is figure out the weakness in those arts and exploit that

i don't know too much about Aikido but i know in Ju Jitsu the chokes, and bone breaking moves all require your opponent to get in grappling range

if you don't let him get in close then you can control him, utilize the weakness of his fighting method and beat him

Judo it's really good when you get in close but if you would like to strike him them, I'm going to say Muay Thai ! ! !

Elbows WILL end most fights and a dominate clinch could put him on the floor as well :)

Wing Chun, it's effective against most martial arts

Tang Su Do, here is the school that I go to...

www.CMMATSD.com