We have a client who owns many rental properties, but all under one
main umbrella.  They own domain names for all of their rental
properties names individually, however those domain names currently
point to the main website page about that particular property.  So,
for example:

Main company: XYZRealty.com
Owned properties: ACondos.com, BCondos.com, CCondos.com

A,B,C all point to their respective pages within the XYZ site as
redirects.  There are probably 20 of these properties.

My question. Is this actually hurting their placement in Google free
results?  Is this considered to be tricking Google or is it not that
big of a deal?  They don't want to invest in individual sites for each
of their properties because its simply not cost effective nor
practical for them when they can have all of their properties on one
main site, yet for marketing purposes having unique domain names that
point people to the one main site is most effective.  I was just
curious to know if having these 20 or so sites pointing there is
detrimental to their free results ranking?

I'll look forward to hearing your thoughts.