If they're in good condition you're doing the right thing, tho not ideal. The Dumor, from what I hear, is rather low quality, the mare&foal levels it out tho. For now.
I'd have a talk with the owner of the pregnant mare what they have in mind for her....where the foal should be born( stall, outside), who's responsible, vet care should something go wrong. Also, the last trimester of pregnancy and the first three months of lactation are critical for the foal, so she should be fed separately and the mare&foal only, and of course grass/hay free choice.