Although 23rd May will be after the peak migration season, I think that you can expect to see some birds.
Some of the shorebirds migrate surprisingly late, for example Grey-tailed Tattler, and these may be present in good numbers. Other shorebirds may not be breeding this year, and could still be present at Mai Po - there are currently still good numbers of sand plovers and stints. Swinhoe's Egrets are still passing through and could be around on your visit, and perhaps even some late landbird migrants (e.g. Arctic Warbler)
Meanwhile, there will be some other birds to see there. In particular, the egretry that has formed this year in Gei wai #14 should be very active at that time, hopefully with lots of chicks to see. There are some Black-winged Stilts and even Avocets looking at possible nest sites now, which you should see. There is still a flock of Black-faced Spoonbills at Mai Po. White-shouldered Starlings are breeding again in the car park, and should be easy to see during the visit.
If your friends are hoping to see huge flocks of thousands of birds you may need to limit their expectations, but if they are happy to see some species that they would not see in the UK, they should still have a good day out on the reserve.
