Here is an example
Link but you can pick these up second hand at virtually any kind of swap meet or used electronics store. You don't have to hook them up to a monitoring service either. Then can send alerts via texts, email, phone call etc. Friend of mine had one installed at his grow cabin in the woods, only person it signaled was him.
P.S. If they have broadband centrally wired in the house then it probably runs by the alarm system which makes using one of these SUPER easy because it just plugs in to an ethernet port.