I'm inclined to agree with others, you need commercial speakers - like DJ speaker or PA speakers, or perhaps good quality studio monitors.
I'm not 100% convinced you need professional installation, but you absolutely need to make sure you don't do a (pardon the expression) half-assed job of it.
This is going to be in a bar which can get rough and tumble. People can get a little unsteady on their feet. You don't want these speakers falling on anyone.
Also, the size and type of the establishment, and the purpose of the speakers. If this is just for background music while while people eat, drink, and converse then several smaller DJ/PA or studio monitor speakers might be better.
However, if this is a public address system as well, or if Karaoke, talent night, or live music is in order, then somewhat larger speakers speakers located near the stage would be better. Though, smaller speaker could still be spread around the bar.
Also, and especially if you use several music speakers, you might want to use a distribution amp. These are similar to a stereo amp, but actually have several channels. For example, an amp might have 8 channels for 4 stereo pairs. These amps aren't cheap, but they are not prohibitively expensive either.
If there might be some PA purpose, as in live music or talent night, then you probably want a conveniently located Stereo PA amp. PA amps are extremely durable, and would very likely be able to drive several pairs of speakers.
Now, if there is a dance floor, and if your friend is in essence making a dance club or disco, the configuration is completely different. The speakers need to be focused on the dance floor, and again, good full-range DJ or PA speaker are necessary.
You can find this type of speaker and people to advice you at local music stores (musical instruments that is). Shops that sell guitars and drums also sell DJ and PA equipment. There are also shops that specialize in suppling DJ's, so consider looking for them. I know I've seen them on-line in the UK. Likely DJ shops can also provide expertise in setting up a dance club.
Though many don't realize it, DJing is a fine art. It is not as simple a matter as just playing music. Frequently a good DJ will transition from one song to another with consideration to pitch or key to make sure the following song is not discordant with the previous. Also, a good DJ keeps up on all the latest music trends and know what tracks will get the crowds moving.
The dance music trends can be very fickle. I remember years ago when 'Ball of Confusion' was hot in all the dance clubs. When that song came on, the club just went nuts. The bartenders would abandon their post, and dance on the top of the bar. Yet, several weeks later the same song came on and barely rated a yawn.
Also, keep in mind that there are several styles of dance club music - techno, jungle, and a dozen more I can't even comprehend. Each style has a loyal following and each style will attracted a different type of crowd.
If your friend is planning a dance club, then he is going to need good professional DJ's, and if he can find them, then he can has the people necessary to advise him on setting up his dance club sound system.
steve/bluewizard