An Australian friend who has a UK working visa has stayed in Spain for 2 months and would like to stay longer, until July -she is staying with a host family, as an au-pair, but this is not considered a job here in Spain-.
My question is whether is legal to go back to UK for a weekend and come back to stay for another 90 days... Would the day counting start from zero or the previous 60 days will count? Will she be able to re-enter Spain? How can she deal with this situation? There is no Australian embassy in Galicia and I don't know where to ask...

No it wouldn't reset the counter - the rule is 90 days in any 180 days.

So it doesn't matter that she leaves the Schengen area... Where can I find more information about this matter? The information given for the UK Spanish Embassy website is insufficient...

Can she get a sponsor visa from her family host?

Also a UK work visa gives her no added advantage as an Australian national in Schengen countries.
She has no choice but to return to UK after another month in Spain and only return after 90 days, i.e. in July.
Alternatively, she can apply for a study visa for Spain, in UK, such as enrolling at a language school, which is separate from Schengen 90-in-180 days rule. She will need to satisfy requirements such as adequate funds, medical insurance and a confirmed place at a school.

<i>Can she get a sponsor visa from her family host?</i>

Only if Spain issues au pair visas. You need to find out from Spanish authorities, or Spanish consulate in UK, where she needs to apply for a her visa, if eligible.

Apparently Spain does issues visas to au pairs from non-EU countries. Get in touch with the Spanish consulate in UK for details on how to apply and what the requirements are. They normally include having enrolled at a language school, minimum salary and pocket money and private medical insurance.