I'm pretty sure the data is encrypted, so if you do a reset it's going to generate a new random key and the data will be permanently lost. Is it possible? Yes, it's possible. The FBI working along with the people who made CellBrite were able to hack that one phone belonging to that terrorist. All at the affordable price tag of $218,000 for one phone (reference: http://money.cnn.com/2016/03/31/technol ... fbi-phone/
Tell your client that if it's worth that much per device, you'll work out a deal to get it done.
I'm sure they'll do it for a bit less now that they've already put the R&D into it. And in case you're wondering, no their commercial tools can't unlock iphones or ipads that have the security set up. I've inquired about this with them several times.