`su -c "Your command right here" -s /bin/sh username`
The above command is correct, but on Red Hat if selinux is enforcing it will not allow cron to execute scripts as another user. example;
execl: couldn't exec /bin/sh
execl: Permission denied
I had to install setroubleshoot and setools and run the following to allow it:
yum install setroubleshoot setools
sealert -a /var/log/audit/audit.log
grep crond /var/log/audit/audit.log | audit2allow -M mypol
semodule -i mypol.p