How about creating a readonly snapshot of the database and giving having them run their ownernight queries against the snapshot. That should give them what they need and give you a database without blocking queries. Lots of things to consider like how/when to create the snapshot and performance, disk space, alter connecting strings and other things but just throwing it out.

https://msdn.microsoft.com/en-ie/library/ms175158(v=sql.100).aspx