Extra Guest in the House

Image from http://thelandlorddiaries.com/?p=86

Yesterday one of my tenants called me to complain about his house-mate who have had a guest staying in the apartment for over 3 months.  The guest is apparently my tenant's brother who is "squatting" in the apartment living room.  While they all do not mind the occasional guest or relative coming to stay over sometimes, a long term guest in the living room is not acceptable.
How to approach this problem?  I wrote an email to my tenant, asking for the guest to leave within 2 weeks.  If he does not reply within 2 days, I will give him a call to discuss.  He has been a good paymaster all this while and the tenancy is due for renewal in 2 months.
Anyone of you have any suggestions for me?
BTW if the squatter look like anything in the photo here, I will send the blogger/property investor/manager eun-yong to settle the problem for me, hahahaha!