Not a problem if they've asked you if that's ok with you. The amount of waste cement should be relatively small but there are ways of cleaning the mixer without pouring the slops on your garden.
1. They can 'sieve' the waste through a sack and take the solid waste away and dispose of it properly at the council tip.
2. If they've done that then the waste water can be poured onto your lawn (if you have one) with your permission, where it won't do any harm.

Leaching isn't really an issue - no worse than concrete foundations, it's just laziness on the part of the builders and if they've got more waste than they should have you could end up uncovering great lumps of concrete next time you try to plant something.

Thanks. The puddle of slops is cement colured but still wet. We are on clay so it's taking a while to drain away but I'll fork it over and hope for the best.

my oh did this and I did as you plan to do ie fork it over. then I picked up any shards of cement and binned them. this was 2 years ago and the soil and the plants under it haven't suffered at all.