If you want to respect the spirit of an ill-formulated rule, then there is no way that you can do this. If instead you are willing to break the rule, provided you don't get caught, read on. But the choice really is all yours.
Any connection you mentioned, the wireless router or the Ad Hoc network, is liable to detection by a simple wifi scan. In either case, the service provider will see only one IP being connected to the network, but she can reveal a new, unknown network and assume someone's not playing by the book. In line of principle, it can even get worse, because it is possible, with the help of a GPS device, to pinpoint the origin of the new wifi network, though this is not very common. Also, there are common techniques (aircrack-ng) to identify which pcs are connected to which APs (Access Points), without any need to connect to either.
You might be able to mitigate the problem by decreasing the transmitting power of both the pcs and the AP (be it wireless router or Ad Hoc network), even though this has other drawbacks, or by choosing the less common 5GHz band. Yet none of this is fool-proof.
The only safe way to allow multiple users would be to connect to the wireless router by cable, for the both of you because there would already be the IP of the router.
Even this requires some adjustment, because the router, upon connection to the home network, identifies itself by MAC address, which carries information about the manufacturer. A savvy tenant could then see connected to her network a component produced by Ciso/Linksys/D-Link/TP-Link, whatever, and would know immediately that her bandwidth is being used by a router plus several connected pcs/phones, ... This can be easily circumvented, because most good routers have a feature called
Clone MAC address allowing them to broadcast a user-defined MAC address, and all you have to do is to chose, for instance, an iPad MAC address.
Still, you may argue that your tenant is not very computer-savvy, otherwise she wouldn't have formulated such a silly rule. What she is trying to do is probably prevent any single user from hogging up all of the bandwidth, a concern which can be addressed with much more sophisticated instrumens (
tc, anyone?). So, if you are willing to take the risk on the basis of this argument, with the proviso stated above about MAC cloning, you might think it safe to implement either option (wireless router/Ad Hoc network), the only distinguishing feature between the two being that the Ad/Hoc network emits a weaker signal, and is thus more unlikely to be detected upon casual perusal (if the perusal is determined, there is no way whatsoever).
Hope this helps.