Do they already have flights arranged and you just need accommodation only or do you need a package holiday?

they have flights they just need cheap accomodation

Have you tried Alpharooms there is a link at the top of the page at Accom only.

Hi, we are going in a private apartment in Calan Forcat in June, we booked it from owners-direct.com. You could also try majorca-accomodation.com they do Menorca as well, I would imagine they will be quite pricey in August though, hope this helps.
Cheers Helen

have a look at this link.

http://www.ownersdirect.co.uk/balearics/B719.htm

we have friends coming to Menorca (cala blanca) who also booked direct via
http://www.villa-holiday.co.uk/area/menorca/menorca_rentals.htm

and got a good price.

Hope this helps