Q: Can you use an old iMac as the main display for mac mini

Can you use an old iMac as the main display for mac mini.  So you are not using the hard drive etc...from the iMac, I ask as my mother in laws iMac has become really slow.  There is nothing wrong with the display, speakers etc..so seems a waste of money to change to a new iMac.  Thoughts welcomed,Thanks.