A local restaurant allows you to bring and consume your own alcohol on its premises. Is this legal?

It is legal provided no charge is made - answers suggesting that corkage charges are legal are wrong

Yes, it is perfectly legal, and saves the restaurant owners the hassle and expense of getting a liquor license.

It's legal.

Your friend could permit his patrons to bring their own as well -- or drink his. If his prices are far higher than outside liquor (typical), he may not get as many liquor sales, but he will still have patrons.

An alternative is for him to have special drinks -- a wide variety of foreign beers or some other angle that might interest patrons.

In the UK it is perfectly legal to not have a liquor licence and encourage customers to bring their own alcohol. Some restaurants charge 'corkage' when you bring your own drinks and that is also legal. Your friend is a liberty to follow them.

Yes, it's perfectly legal and common practice. The restaurants sometimes make a small charge (Corkage fee) for opening the bottle and the use of glasses.

It is entirely legal, provided those who bring and consume alcohol are above the legal age to do so.

Some places allow that, yes. They are usually known as BYOB restaurants.

Maybe your friend should lower his prices.

Perfectly legal.

Seems your friend does not know how to handle competition.

yes - its perfectly legal