With most Dell "Business" targeted laptops there is an additional security feature in addition to the BIOS and OS related passwords. This password prompt appears before the BIOS password and can sometimes go wrong and stop a valid user from accessing the device. I'm not sure of this being used with a tablet but I can see the "sense" of protecting Tablet devices this way. The option is usually switched off but can sometimes switch itself on (Not sure what is the cause of this but I have seen it happen)
The password for this system lock is assigned by Dell at the factory, and should be in the documentation that came with the device. Of course many users will not have this information, either because they have misplaced it, or it was not passed on with a used device. Unfortunately the only way to retrieve this password is limited to Dell themselves, the user should contact Dell with all the information they have. The Service Tag and the original order are usually quoted but in the case of a second user other information may be accepted. There are a few independent IT technician that have access to the software required but Dell are the best solution.
Hopefully you have a bios password issue rather than the additional security, and you may be able to reset the BIOS by removing the RTC battery, (difficult enough with a laptop and even more so with a tablet