She will be 'allowed' to ride anything she wants to ride, but warning signs will help guide her away from the attractions that really aren't suited to pregnant women (for various reasons).
The Safari is very bumpy and jerky in places (sometimes more so, sometimes less so), making it a slight risk for Baby. Other attraction restrictions may be less obvious (the ride vehicle may put Mom in an awkward position, cutting down on the blood/oxygen supply to Baby, etc), so it becomes an individual decision as to whether or not she wants to ride. Disney CM's won't stop her, but the warnings are (IMO) a good guage as to what's ok and what's not.