Will a shock collar be effective for my chihuahua's behavior?  

Please do not use a shock collar on a dog. I think the more effective way to stop the chihuahuas be behavior is for you to stop it yourself. Say if you all are in the room with the dogs present and the chihuahua moves to attack the pug your movement should be to snap your fingers (if you can do it real loud) or make a shh sound and quickly move the chihuahua away. You'll then want to "ignore" the dog and if it tries again do the same thing. You and your friend should both alternate doing this. It won't happen like magic for the chihuahua to stop but if you're consistent in it the dog will understand that the masters do not appreciate what's going on. You may even have to constantly be on alert but avoid getting "excited" such as high pitch voices or yelling. Dogs don't yell when they are upset they respond using movement and low sounds. reacting in high noises can undo anything that you may have previously tried to do.

doubt it. shock collars are already a controversial product, but especially on a dog as small as a chihuahua i'd advise you NOT to use one.

chihuahua's are just pissy dogs. maybe it's to make up for their small size, idk. it's not really a behavioral problem with them. their stubbornness and grouchiness is just part of the breed.

how old is the chihuahua, and how long has your friend had him?
is the pug a new addition?
if that's the case, it's going to be hard, just because chihuahuas usually don't like new dogs and/or people.

A shock collar sounds cruel to me. I used to use time outs when I had dogs with bad attitudes in the past, and removal of their treats or playtimes.

Nowadays, I'm lucky, I have 2 little sweethearts, and have never had to discipline them due to attitude.

I hate the things, but they work. If you place it on your neck and push the button you will see what I mean. It gives you a shock right to your privates..You can adjust the strength. Dangerous , check with your vet..