Your friend is well...dumb. Male bettas will fight and can do some serious damage to one another. One of our members did succesfully house a couple of males with females in VERY large tank. The general rule for keeping male bettas together is to give each male 3 feet in every direction.....even then I don't reccomend even attempting to house them together.
Every male needs to be seperated in his own tank. Females might (but not always) be able to be housed together in 10 gallons with lots of hiding places.
Who am I? Well I am a Junior Member of the International Betta Congress and a breeder of white HMPKs, red dragon HMPKs, black/red dragon HMPKs, metallic multi HMPKs, blue masked HMPKs, and now multi color HMs. Soon to add black HMs. I will be showing in New Breeder class in fall of 2012.