This probably the stupidest and most inefficient way to manage an OPAC. First of all, why shouldn't she be able to reserve the book? Flag it, once it is back to the main library you send a message to the patreon that the book is available for pick up; at least, that's how we do it in civilized countries.
Second, why the heck should you hide the books? If they are on the mobile library, write that they are there, so people know where to find them.
Third, if you have resources for consultation only, flag them "for consultation only", not "on loan"; again, that way people know what is available and where.