In Spanish, is there a rule for when numerals change for gender or do you just have to memorize each one?

The only time a number would ever change gender is if it's 'one'.

Also, numbers like three hundred (trescientos), four hundred(cuatrocientos) will change depending on gender. Also, what your friend said is true.

5 women, 1 man = ellos, not ellas

another thing, the number will change gender if it's a number that's one of something..like thirty one, forty one, etc.

tres hombres

cinco mujeres

Un and una can represent the number one, but they also mean "a" in Spanish. Articles, like "a" and "the" in Spanish change according to gender. However, numbers don't. If you listen to some Spanish-speakers who are learning English for the first time, they also get this confused and always use the word "one" instead of "a". So, to sum it up, un, uno, and una can all be forms of one (changing gender when the word can be used interchangeably with the word "a", but other numbers do not change.

As for groups and what "gender" to assign them, your friend is correct. One man or masculine object changes the group to masculine.

In Spanish, if twenty women and one man are inside bus, the group will be mentioned as passengers (male). It is will be mentioned only as feminine passengers gender when the group goes only of women.