Whoops - this happened to me and my garage mice. For a few years I deceived my husband into thinking they had gone (they were feeding on the sack of birdseed I kept in there) by putting down six peanuts each night and then getting up before him, going into the garage and replacing the nuts the mice had eaten in the night

Eventually, of course, the mice found there way up through the garage ceiling (through the little holes around the pipes) and ran riot in between the floors. They sounded like a herd of elephants and in the night their chomping sounded just like a huge rat was trying to eat its way through the bedroom ceiling. I had to confess

to my nut replacement and I'm afraid the traps went up into the loft where they had finally managed to find there way to. Nobody told me though - but there were furtive nods and winks between my husband and my son each time my son heard a trap go off in the loft. I removed the birdseed from the garage and as far as I can tell we have no more mice.
Your friend wont get rid of the mice if she continues to feed them. It is tough stopping feeding them this time of year with the cold winter weather coming along soon. Mice are not an RSPCA problem I'm afraid. If they are really tiny they could be shrews - do they have long noses?
Am still thinking of a solution. Is the greenhouse quite a way from the house? If she has had a mouse in her greenhouse for two years it is strange that there are only three - there should be 3000 by now!
Taz x