Just simply let him know that uromastyx do very poorly
in high humidity conditions. There is a high risk of both
bacterial and fungal disease such as the dreaded blister
disease not to mention tail rot. There is also a good
chance that they will develop an upper respiratory
infection due to the increased humidity.
Uromastyx are for the most part desert animals and many
never encounter water whether it be rain, lakes or rivers.
They are mostly hydrated by the food they eat. Since most
captive bred/born or captive housed are fed extremely
high water content greens and veggies, there is no reason
to offer a healthy uro any water at all. Especially a large
bowl which they would probably walk/run through so as
to end up with water between/under their scales and tail
whorls which would be a good candidate for the above
mentioned diseases. If you like, print this and show it to
the pet store owner.
I've been working with uromastyx primarily as a keeper
and breeder for over a dozen years so I do know a little
about what I am saying. I am also a recently retired vet
tech and have seen my share of these dreaded and hard
to cure fungal and bacterial diseases.
As to the "giant black" lizards:
A good rule of thumb is to keep different sub/species
to themselves. I'm not really sure what you mean by a
"giant black lizard" as I've never heard of the U a
aegypticus or U a microlepis referred to by this name.
John